diff --git a/server/src/tools/def_navigation.py b/server/src/tools/def_navigation.py
new file mode 100644
index 0000000..b04bb45
--- /dev/null
+++ b/server/src/tools/def_navigation.py
@@ -0,0 +1,191 @@
+"""Navigation between Tcl code and the block templates and addresses of .def files."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import lsprotocol.types as lsp
+
+from tools.def_symbols import ADDRESS, BLOCK_TEMPLATE, FORMAT, DefDeclaration, DefDocument
+from tools.navigation import FileSymbolIndex, SymbolOccurrence
+
+DefTarget = tuple[str, str]
+_KIND_LABELS = {BLOCK_TEMPLATE: "Block template", ADDRESS: "Address", FORMAT: "Format"}
+# Address properties in display order; others follow as declared.
+_ADDRESS_PROPERTIES = (
+    ("FORMAT", "Format"),
+    ("LEADER", "Leader"),
+    ("TRAILER", "Trailer"),
+    ("MIN", "Min"),
+    ("MAX", "Max"),
+    ("FORCE", "Modality"),
+    ("ZERO_FORMAT", "Zero format"),
+    ("INCREMENTAL", "Incremental"),
+    ("OMIT", "Omit"),
+)
+_MODALITY = {
+    "OFF": "modal, output only on change",
+    "ONCE": "output once, then modal",
+    "ALWAYS": "non-modal, always output",
+}
+
+
+def _range(line: int, start: int, end: int) -> lsp.Range:
+    return lsp.Range(
+        start=lsp.Position(line=line, character=start),
+        end=lsp.Position(line=line, character=end),
+    )
+
+
+def _contains(line: int, start: int, end: int, position: lsp.Position) -> bool:
+    return position.line == line and start <= position.character <= end
+
+
+def def_symbol_at(document: DefDocument, position: lsp.Position) -> tuple[DefTarget, lsp.Range, bool] | None:
+    """Return the block template or address at ``position`` of a .def document.
+
+    The flag tells whether the position is on a declaration.
+    """
+    for declaration in document.declarations:
+        if declaration.kind != FORMAT and _contains(declaration.line, declaration.start, declaration.end, position):
+            return (declaration.kind, declaration.name), _range(declaration.line, declaration.start, declaration.end), True
+    for reference in document.references:
+        if _contains(reference.line, reference.start, reference.end, position):
+            return (reference.kind, reference.name), _range(reference.line, reference.start, reference.end), False
+    return None
+
+
+def _uri(path: str, uris: dict[str, str]) -> str:
+    return uris.get(path) or Path(path).as_uri()
+
+
+def def_declarations(documents: dict[str, DefDocument], target: DefTarget) -> list[tuple[str, DefDeclaration]]:
+    kind, name = target
+    return [
+        (path, declaration)
+        for path, document in documents.items()
+        for declaration in document.declarations
+        if declaration.kind == kind and declaration.name == name
+    ]
+
+
+def def_definition_locations(
+    documents: dict[str, DefDocument], target: DefTarget, uris: dict[str, str] | None = None
+) -> list[lsp.Location]:
+    uris = uris or {}
+    return [
+        lsp.Location(uri=_uri(path, uris), range=_range(declaration.line, declaration.start, declaration.end))
+        for path, declaration in def_declarations(documents, target)
+    ]
+
+
+def def_reference_locations(
+    documents: dict[str, DefDocument],
+    target: DefTarget,
+    include_declaration: bool,
+    uris: dict[str, str] | None = None,
+) -> list[lsp.Location]:
+    """Return .def occurrences: declarations and addresses used in block templates."""
+    kind, name = target
+    uris = uris or {}
+    locations = def_definition_locations(documents, target, uris) if include_declaration else []
+    for path, document in documents.items():
+        for reference in document.references:
+            if reference.kind == kind and reference.name == name:
+                locations.append(
+                    lsp.Location(uri=_uri(path, uris), range=_range(reference.line, reference.start, reference.end))
+                )
+    return locations
+
+
+def tcl_def_occurrences(
+    indexes: dict[str, FileSymbolIndex], target: DefTarget
+) -> list[tuple[FileSymbolIndex, SymbolOccurrence]]:
+    kind, name = target
+    return [
+        (index, occurrence)
+        for index in indexes.values()
+        for occurrence in index.occurrences
+        if occurrence.identity.kind == kind and occurrence.identity.name == name
+    ]
+
+
+def all_def_target_locations(
+    documents: dict[str, DefDocument],
+    indexes: dict[str, FileSymbolIndex],
+    target: DefTarget,
+    include_declaration: bool,
+    uris: dict[str, str] | None = None,
+) -> list[lsp.Location]:
+    locations = def_reference_locations(documents, target, include_declaration, uris)
+    locations.extend(
+        lsp.Location(uri=index.uri, range=occurrence.range) for index, occurrence in tcl_def_occurrences(indexes, target)
+    )
+    unique = {}
+    for location in locations:
+        key = (location.uri, location.range.start.line, location.range.start.character)
+        unique.setdefault(key, location)
+    return sorted(unique.values(), key=lambda item: (item.uri, item.range.start.line, item.range.start.character))
+
+
+def def_rename_edits(
+    documents: dict[str, DefDocument],
+    indexes: dict[str, FileSymbolIndex],
+    target: DefTarget,
+    new_name: str,
+    uris: dict[str, str] | None = None,
+) -> lsp.WorkspaceEdit | None:
+    """Rename a block template or address in all .def and Tcl files, if it is declared."""
+    if not def_declarations(documents, target):
+        return None
+    changes: dict[str, list[lsp.TextEdit]] = {}
+    for location in all_def_target_locations(documents, indexes, target, True, uris):
+        changes.setdefault(location.uri, []).append(lsp.TextEdit(range=location.range, new_text=new_name))
+    for edits in changes.values():
+        edits.sort(key=lambda edit: (edit.range.start.line, edit.range.start.character), reverse=True)
+    return lsp.WorkspaceEdit(changes=changes)
+
+
+def _escape_cell(value: str) -> str:
+    return value.replace("|", "\\|") or " "
+
+
+def _address_table(declaration: DefDeclaration, formats: dict[str, DefDeclaration]) -> str:
+    properties = dict(declaration.properties)
+    rows = []
+    for key, label in _ADDRESS_PROPERTIES:
+        value = properties.pop(key, None)
+        if value is None:
+            continue
+        cell = f"`{value}`" if value else ""
+        if key == "FORMAT" and value in formats:
+            cell += f" → `{dict(formats[value].properties).get('FORMAT', '')}`"
+        if key == "FORCE":
+            cell += f" ({_MODALITY[value.upper()]})" if value.upper() in _MODALITY else ""
+        rows.append(f"| {label} | {_escape_cell(cell)} |")
+    rows.extend(f"| {key.title()} | {_escape_cell(f'`{value}`' if value else '')} |" for key, value in properties.items())
+    if not rows:
+        return "_No properties_"
+    return "\n".join(["| Property | Value |", "|---|---|", *rows])
+
+
+def def_hover_markdown(documents: dict[str, DefDocument], target: DefTarget) -> str | None:
+    declarations = def_declarations(documents, target)
+    if not declarations:
+        return None
+    formats = {
+        declaration.name: declaration
+        for document in documents.values()
+        for declaration in document.declarations
+        if declaration.kind == FORMAT
+    }
+    kind, name = target
+    sections = []
+    for path, declaration in declarations:
+        header = f"**{_KIND_LABELS[kind]}** `{name}` — {Path(path).name}:{declaration.line + 1}"
+        if kind == ADDRESS:
+            body = _address_table(declaration, formats)
+        else:
+            body = f"```def\n{declaration.text}\n```"
+        sections.append(f"{header}\n\n{body}")
+    return "\n\n---\n\n".join(sections)
diff --git a/server/tests/python_tests/test_def_navigation.py b/server/tests/python_tests/test_def_navigation.py
new file mode 100644
index 0000000..4ac219a
--- /dev/null
+++ b/server/tests/python_tests/test_def_navigation.py
@@ -0,0 +1,242 @@
+"""Go to Definition, hover, references and rename between Tcl and .def files."""
+
+from collections import namedtuple
+from pathlib import Path
+
+import lsprotocol.types as lsp  # type: ignore
+from pygls.workspace import Workspace
+
+import lsp_server
+from lsp_tclserver import TclLanguageServer
+from tools.def_symbols import ADDRESS, BLOCK_TEMPLATE, FORMAT, parse_def_document
+
+PSC = """<?xml version="1.0" encoding="UTF-8"?>
+<Configuration>
+  <Sourcing><Sequence>
+    <Layer Name="Service" SubFolder="service">
+      <DefinedEvents><Filename Name="service"/></DefinedEvents>
+    </Layer>
+  </Sequence></Sourcing>
+</Configuration>
+"""
+
+DEF = """MACHINE Default
+
+FORMATTING
+{
+    FORMAT Coordinate "&__4.3_"
+    ADDRESS SPOS
+    {
+        FORMAT      Coordinate
+        FORCE       off
+        MAX         99999.999 Abort
+        MIN         -99999.999 Abort
+        LEADER      "SPOS="
+    }
+    # ADDRESS commented_out
+    BLOCK_TEMPLATE steady_rest
+    {
+        SPOS[$mom_pos(0)]
+        Text[M60]\\opt
+    }
+}
+"""
+
+TCL = """proc MOM_steady {} {
+    MOM_do_template steady_rest
+    MOM_force Once SPOS X
+    MOM_ask_address_value "SPOS"
+    set name steady_rest
+}
+"""
+
+
+def _project(tmp_path: Path, monkeypatch):
+    (tmp_path / "service").mkdir()
+    (tmp_path / "post.psc").write_text(PSC, encoding="utf-8")
+    def_file = tmp_path / "service" / "service.def"
+    def_file.write_text(DEF, encoding="utf-8")
+    tcl_file = tmp_path / "caller.tcl"
+    tcl_file.write_text(TCL, encoding="utf-8")
+
+    server = TclLanguageServer(name="def-navigation-test", version="1", max_workers=1)
+    server.protocol._workspace = Workspace(  # pylint: disable=protected-access
+        root_uri=tmp_path.as_uri(),
+        sync_kind=lsp.TextDocumentSyncKind.Incremental,
+        workspace_folders=[lsp.WorkspaceFolder(uri=tmp_path.as_uri(), name="root")],
+        position_encoding=lsp.PositionEncodingKind.Utf16,
+    )
+    monkeypatch.setattr(lsp_server, "LSP_SERVER", server)
+    server.refresh_psc_scripts([tmp_path])
+    server.workspace.put_text_document(
+        lsp.TextDocumentItem(uri=tcl_file.as_uri(), language_id="tcl", version=1, text=TCL)
+    )
+    server.update_poco_completion_for_file(server.workspace.get_text_document(tcl_file.as_uri()))
+    return server, def_file, tcl_file
+
+
+def _position(source: str, needle: str, occurrence: int = 0, offset: int = 1) -> lsp.Position:
+    index = -1
+    for _ in range(occurrence + 1):
+        index = source.index(needle, index + 1)
+    line = source.count("\n", 0, index)
+    column = index - (source.rfind("\n", 0, index) + 1)
+    return lsp.Position(line=line, character=column + offset)
+
+
+def _tcl_params(tcl_file: Path, needle: str, occurrence: int = 0):
+    return lsp.TextDocumentIdentifier(uri=tcl_file.as_uri()), _position(TCL, needle, occurrence)
+
+
+# The client sends custom .def requests as plain JSON; pygls exposes them as namedtuples.
+_Doc = namedtuple("Object", ["uri"])
+_Pos = namedtuple("Object", ["line", "character"])
+_Params = namedtuple("Object", ["textDocument", "position", "text", "includeDeclaration", "newName"])
+
+
+def _def_params(def_file: Path, needle: str, occurrence: int = 0, text: str = DEF, offset: int = 1, **extra):
+    position = _position(text, needle, occurrence, offset)
+    return _Params(
+        _Doc(def_file.as_uri()),
+        _Pos(position.line, position.character),
+        text,
+        extra.get("includeDeclaration", True),
+        extra.get("newName", ""),
+    )
+
+
+def _lines(locations):
+    return sorted((Path(location.uri).name, location.range.start.line, location.range.start.character) for location in locations)
+
+
+def test_parse_def_document_declarations_and_references():
+    document = parse_def_document(DEF)
+    kinds = [(item.kind, item.name) for item in document.declarations]
+    assert kinds == [(FORMAT, "Coordinate"), (ADDRESS, "SPOS"), (BLOCK_TEMPLATE, "steady_rest")]
+    address = document.declarations[1]
+    assert (address.line, address.start, address.end) == (5, 12, 16)
+    assert dict(address.properties)["LEADER"] == '"SPOS="'
+    assert [(ref.name, ref.line, ref.container) for ref in document.references] == [
+        ("SPOS", 16, "steady_rest"),
+        ("Text", 17, "steady_rest"),
+    ]
+    assert document.declarations[2].text.splitlines()[-1].strip() == "}"
+
+
+def test_tcl_goto_definition_of_template_and_address(tmp_path, monkeypatch):
+    _, def_file, tcl_file = _project(tmp_path, monkeypatch)
+    document, position = _tcl_params(tcl_file, "steady_rest")
+    result = lsp_server.goto_definition(lsp.DefinitionParams(text_document=document, position=position))
+    assert _lines(result) == [("service.def", 14, 19)]
+
+    for needle, occurrence in (("SPOS", 0), ("SPOS", 1)):
+        document, position = _tcl_params(tcl_file, needle, occurrence)
+        result = lsp_server.goto_definition(lsp.DefinitionParams(text_document=document, position=position))
+        assert _lines(result) == [("service.def", 5, 12)]
+
+
+def test_tcl_goto_definition_ignores_plain_words_and_unknown_names(tmp_path, monkeypatch):
+    _, _, tcl_file = _project(tmp_path, monkeypatch)
+    # "set name steady_rest" is no template argument.
+    document, position = _tcl_params(tcl_file, "steady_rest", 1)
+    assert lsp_server.goto_definition(lsp.DefinitionParams(text_document=document, position=position)) is None
+    document, position = lsp.TextDocumentIdentifier(uri=tcl_file.as_uri()), _position(TCL, "SPOS X", offset=5)
+    assert lsp_server.goto_definition(lsp.DefinitionParams(text_document=document, position=position)) is None
+
+
+def test_tcl_hover_shows_template_body_and_address_properties(tmp_path, monkeypatch):
+    _, _, tcl_file = _project(tmp_path, monkeypatch)
+    document, position = _tcl_params(tcl_file, "steady_rest")
+    hover = lsp_server.hover(lsp.HoverParams(text_document=document, position=position))
+    assert "Block template" in hover.contents.value
+    assert "SPOS[$mom_pos(0)]" in hover.contents.value
+    assert "```def" in hover.contents.value
+
+    document, position = _tcl_params(tcl_file, "SPOS")
+    value = lsp_server.hover(lsp.HoverParams(text_document=document, position=position)).contents.value
+    assert "| Format | `Coordinate` → `\"&__4.3_\"` |" in value
+    assert '| Leader | `"SPOS="` |' in value
+    assert "| Min | `-99999.999 Abort` |" in value
+    assert "| Max | `99999.999 Abort` |" in value
+    assert "| Modality | `off` (modal, output only on change) |" in value
+
+
+def test_tcl_references_include_def_declaration_and_template_elements(tmp_path, monkeypatch):
+    _, _, tcl_file = _project(tmp_path, monkeypatch)
+    document, position = _tcl_params(tcl_file, "SPOS")
+    result = lsp_server.references(
+        lsp.ReferenceParams(
+            text_document=document,
+            position=position,
+            context=lsp.ReferenceContext(include_declaration=True),
+        )
+    )
+    assert _lines(result) == [
+        ("caller.tcl", 2, 19),
+        ("caller.tcl", 3, 27),
+        ("service.def", 5, 12),
+        ("service.def", 16, 8),
+    ]
+    result = lsp_server.references(
+        lsp.ReferenceParams(
+            text_document=document,
+            position=position,
+            context=lsp.ReferenceContext(include_declaration=False),
+        )
+    )
+    assert ("service.def", 5, 12) not in _lines(result)
+
+
+def test_tcl_rename_updates_def_and_tcl(tmp_path, monkeypatch):
+    _, def_file, tcl_file = _project(tmp_path, monkeypatch)
+    document, position = _tcl_params(tcl_file, "SPOS")
+    prepared = lsp_server.prepare_rename(lsp.PrepareRenameParams(text_document=document, position=position))
+    assert prepared.placeholder == "SPOS"
+
+    edit = lsp_server.rename(lsp.RenameParams(text_document=document, position=position, new_name="STEADY_POS"))
+    edits = {Path(uri).name: [(e.range.start.line, e.range.start.character) for e in items] for uri, items in edit.changes.items()}
+    assert edits == {"caller.tcl": [(3, 27), (2, 19)], "service.def": [(16, 8), (5, 12)]}
+    assert all(e.new_text == "STEADY_POS" for items in edit.changes.values() for e in items)
+
+
+def test_undeclared_names_are_not_renamed(tmp_path, monkeypatch):
+    _, _, tcl_file = _project(tmp_path, monkeypatch)
+    document = lsp.TextDocumentIdentifier(uri=tcl_file.as_uri())
+    position = _position(TCL, "SPOS X", offset=5)
+    assert lsp_server.prepare_rename(lsp.PrepareRenameParams(text_document=document, position=position)) is None
+
+
+def test_def_requests_resolve_declarations_and_elements(tmp_path, monkeypatch):
+    _, def_file, _ = _project(tmp_path, monkeypatch)
+    # Address element inside a block template -> ADDRESS declaration.
+    result = lsp_server.def_definition(_def_params(def_file, "SPOS[", offset=1))
+    assert _lines(result) == [("service.def", 5, 12)]
+
+    hover = lsp_server.def_hover(_def_params(def_file, "steady_rest"))
+    assert "Text[M60]" in hover.contents.value
+
+    references = lsp_server.def_references(_def_params(def_file, "ADDRESS SPOS", offset=9))
+    assert _lines(references) == [
+        ("caller.tcl", 2, 19),
+        ("caller.tcl", 3, 27),
+        ("service.def", 5, 12),
+        ("service.def", 16, 8),
+    ]
+    assert lsp_server.def_hover(_def_params(def_file, "MACHINE")) is None
+
+
+def test_def_requests_use_unsaved_text(tmp_path, monkeypatch):
+    _, def_file, _ = _project(tmp_path, monkeypatch)
+    text = DEF.replace("BLOCK_TEMPLATE steady_rest", "BLOCK_TEMPLATE steady_rest_new")
+    params = _def_params(def_file, "steady_rest_new", text=text, newName="rest")
+    assert lsp_server.def_prepare_rename(params).placeholder == "steady_rest_new"
+    edit = lsp_server.def_rename(params)
+    assert list(edit.changes) == [def_file.as_uri()]
+
+
+def test_def_rename_updates_tcl_callers(tmp_path, monkeypatch):
+    _, def_file, _ = _project(tmp_path, monkeypatch)
+    edit = lsp_server.def_rename(_def_params(def_file, "steady_rest", newName="lunette"))
+    edits = {Path(uri).name: [(e.range.start.line, e.range.start.character) for e in items] for uri, items in edit.changes.items()}
+    assert edits == {"caller.tcl": [(1, 20)], "service.def": [(14, 19)]}
+    assert lsp_server.def_rename(_def_params(def_file, "steady_rest", newName="bad name")) is None