diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3085d96..500caf5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+- Add signature help for custom TCL procedures and built-in NX/MOM procedures
+
 ## [0.0.1]
 
 - Initial release
diff --git a/README.md b/README.md
index 26d882a..068b5bb 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@ A comprehensive VS Code extension providing language support for NX CAM postproc
 - **Multi-language Support** - Supports NX CDL, TCL, and DEF file formats
 - **Intelligent Code Analysis** - Linting and error detection for postprocessor code
 - **Auto-completion** - Context-aware code completion for faster development
+- **Signature Help** - Shows parameters and documentation for custom and NX procedures
 
 ## Supported File Types
 
@@ -40,6 +41,7 @@ Simply open any supported file type and enjoy:
 - Code completion
 - Code formatting (Format Document command)
 - Hover information
+- Signature help while entering procedure arguments
 
 ## Contributing
 
diff --git a/server/src/lsp_server.py b/server/src/lsp_server.py
index 026568e..2fc7e45 100644
--- a/server/src/lsp_server.py
+++ b/server/src/lsp_server.py
@@ -46,6 +46,7 @@ from tools.folding_ranges import build_folding_ranges
 from tools.semantic_tokens import _Highlighter, TOKEN_TYPES, TokenModifier
 from tools.completion_items import completion, remove_existing_items, remove_shared_keys
 from tools.inlay_hint import InlayHintGenerator
+from tools.signature_help import build_signature_help
 from tools.file_sourcing import get_all_psc_files, read_psc_file
 from lsp_tclserver import TclLanguageServer
 
@@ -182,6 +183,43 @@ def on_completion(params: lsp.CompletionParams) -> lsp.CompletionList:
     return lsp.CompletionList(is_incomplete=False, items=merged)
 
 
+@LSP_SERVER.feature(
+    lsp.TEXT_DOCUMENT_SIGNATURE_HELP,
+    lsp.SignatureHelpOptions(
+        trigger_characters=[" "],
+        retrigger_characters=[" "],
+    ),
+)
+def signature_help(params: lsp.SignatureHelpParams) -> lsp.SignatureHelp | None:
+    document = LSP_SERVER.workspace.get_text_document(params.text_document.uri)
+    tree = LSP_SERVER.get_tree(document)
+
+    filepath = str(pathlib.Path(uris.to_fs_path(document.uri)))
+    custom_signatures: dict[str, list[str]] = {}
+    custom_docs: dict[str, str] = {}
+
+    # Prefer declarations from the current document if duplicate proc names
+    # exist in the workspace.
+    for indexed_path, signatures in LSP_SERVER.proc_signatures.items():
+        if indexed_path != filepath:
+            custom_signatures.update(signatures)
+    custom_signatures.update(LSP_SERVER.proc_signatures.get(filepath, {}))
+
+    for indexed_path, docs in LSP_SERVER.proc_docs.items():
+        if indexed_path != filepath:
+            custom_docs.update(docs)
+    custom_docs.update(LSP_SERVER.proc_docs.get(filepath, {}))
+
+    return build_signature_help(
+        document.source,
+        tree,
+        params.position,
+        custom_signatures,
+        custom_docs,
+        standard_items.json_data.get("MOM_procs", []),
+    )
+
+
 # @LSP_SERVER.feature(lsp.TEXT_DOCUMENT_DOCUMENT_SYMBOL)
 # def document_symbols(params: lsp.DocumentSymbolParams):
 #     doc = LSP_SERVER.workspace.get_text_document(params.text_document.uri)
diff --git a/server/src/tools/signature_help.py b/server/src/tools/signature_help.py
new file mode 100644
index 0000000..54ca308
--- /dev/null
+++ b/server/src/tools/signature_help.py
@@ -0,0 +1,173 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+import lsprotocol.types as lsp
+from tclint.syntax_tree import Command, Node
+
+
+@dataclass(frozen=True)
+class ActiveCall:
+    name: str
+    active_argument: int
+
+
+def _to_lsp_position(position: tuple[int, int] | None) -> tuple[int, int] | None:
+    if position is None:
+        return None
+    return position[0] - 1, position[1] - 1
+
+
+def _contains_cursor(
+    command: Command, source_lines: list[str], cursor: tuple[int, int]
+) -> bool:
+    start = _to_lsp_position(getattr(command, "pos", None))
+    end = _to_lsp_position(getattr(command, "end_pos", None))
+    if start is None or end is None or cursor < start:
+        return False
+
+    if cursor <= end:
+        return True
+
+    # tclint excludes trailing whitespace from a command's range. Keep the
+    # command active while the cursor is in that whitespace so typing a space
+    # after the command name or an argument can trigger signature help.
+    if cursor[0] != end[0] or cursor[0] >= len(source_lines):
+        return False
+
+    line = source_lines[cursor[0]]
+    if cursor[1] > len(line):
+        return False
+
+    return line[end[1] : cursor[1]].isspace()
+
+
+def _active_argument(command: Command, cursor: tuple[int, int]) -> int:
+    for index, argument in enumerate(command.args):
+        start = _to_lsp_position(getattr(argument, "pos", None))
+        end = _to_lsp_position(getattr(argument, "end_pos", None))
+        if start is None or end is None:
+            continue
+        if cursor < start or start <= cursor <= end:
+            return index
+
+    return len(command.args)
+
+
+def find_active_call(
+    source: str, tree: Node, position: lsp.Position
+) -> ActiveCall | None:
+    """Return the innermost Tcl command at the cursor and its argument index."""
+    cursor = (position.line, position.character)
+    source_lines = source.split("\n")
+    candidates: list[tuple[int, tuple[int, int], Command]] = []
+
+    def walk(node: Node, depth: int = 0) -> None:
+        if isinstance(node, Command) and _contains_cursor(node, source_lines, cursor):
+            start = _to_lsp_position(getattr(node, "pos", None)) or (0, 0)
+            candidates.append((depth, start, node))
+
+        for child in getattr(node, "children", []):
+            walk(child, depth + 1)
+
+    walk(tree)
+    if not candidates:
+        return None
+
+    _, _, command = max(candidates, key=lambda item: (item[0], item[1]))
+    name = getattr(command.routine, "contents", None)
+    if not name:
+        return None
+
+    return ActiveCall(name=name, active_argument=_active_argument(command, cursor))
+
+
+def _parameter_label(
+    signature_label: str, name: str, start_at: int = 0
+) -> str | tuple[int, int]:
+    start = signature_label.find(name, start_at)
+    if start < 0:
+        start = signature_label.lower().find(name.lower(), start_at)
+    if start < 0:
+        return name
+    return start, start + len(name)
+
+
+def _custom_signature(
+    name: str, parameter_names: list[str], documentation: str | None
+) -> lsp.SignatureInformation:
+    label = " ".join([name, *parameter_names])
+    parameters = []
+    search_from = len(name)
+    for parameter_name in parameter_names:
+        parameter_label = _parameter_label(label, parameter_name, search_from)
+        parameters.append(lsp.ParameterInformation(label=parameter_label))
+        if isinstance(parameter_label, tuple):
+            search_from = parameter_label[1]
+    return lsp.SignatureInformation(
+        label=label,
+        documentation=(
+            lsp.MarkupContent(kind=lsp.MarkupKind.Markdown, value=documentation)
+            if documentation
+            else None
+        ),
+        parameters=parameters,
+    )
+
+
+def _builtin_signature(item: dict[str, Any]) -> lsp.SignatureInformation:
+    label = item.get("format") or item.get("label", "")
+    parameters = []
+    for parameter in item.get("parameters", []):
+        name = parameter.get("name", "")
+        parameters.append(
+            lsp.ParameterInformation(
+                label=_parameter_label(label, name),
+                documentation=parameter.get("desc") or None,
+            )
+        )
+
+    return lsp.SignatureInformation(
+        label=label,
+        documentation=item.get("description") or None,
+        parameters=parameters,
+    )
+
+
+def build_signature_help(
+    source: str,
+    tree: Node,
+    position: lsp.Position,
+    custom_signatures: dict[str, list[str]],
+    custom_docs: dict[str, str],
+    builtin_items: list[dict[str, Any]],
+) -> lsp.SignatureHelp | None:
+    call = find_active_call(source, tree, position)
+    if call is None:
+        return None
+
+    if call.name in custom_signatures:
+        signature = _custom_signature(
+            call.name,
+            custom_signatures[call.name],
+            custom_docs.get(call.name),
+        )
+    else:
+        item = next(
+            (item for item in builtin_items if item.get("label") == call.name), None
+        )
+        if item is None:
+            return None
+        signature = _builtin_signature(item)
+
+    parameter_count = len(signature.parameters or [])
+    active_parameter = (
+        min(call.active_argument, parameter_count - 1) if parameter_count else None
+    )
+    signature.active_parameter = active_parameter
+    return lsp.SignatureHelp(
+        signatures=[signature],
+        active_signature=0,
+        active_parameter=active_parameter,
+    )
diff --git a/server/tests/python_tests/test_signature_help.py b/server/tests/python_tests/test_signature_help.py
new file mode 100644
index 0000000..7fd4729
--- /dev/null
+++ b/server/tests/python_tests/test_signature_help.py
@@ -0,0 +1,92 @@
+import sys
+from pathlib import Path
+
+
+THIS_DIR = Path(__file__).parent
+SRC_DIR = THIS_DIR.parent.parent / "src"
+if str(SRC_DIR) not in sys.path:
+    sys.path.insert(0, str(SRC_DIR))
+
+import lsprotocol.types as lsp  # type: ignore
+
+from common.load_data import standard_items
+from tools.parser import CustomParser
+from tools.signature_help import build_signature_help, find_active_call
+
+
+CUSTOM_SIGNATURES = {
+    "SERVICE_spacer_output": ["type", "length", "line_num", "output"]
+}
+
+
+def test_custom_proc_signature_and_active_parameter():
+    source = 'SERVICE_spacer_output "*" '
+    tree = CustomParser().parse(source)
+
+    result = build_signature_help(
+        source,
+        tree,
+        lsp.Position(line=0, character=len(source)),
+        CUSTOM_SIGNATURES,
+        {"SERVICE_spacer_output": "Outputs a spacer line."},
+        [],
+    )
+
+    assert result is not None
+    assert result.active_parameter == 1
+    signature = result.signatures[0]
+    assert signature.label == "SERVICE_spacer_output type length line_num output"
+    assert (  # type: ignore[union-attr]
+        signature.documentation.value == "Outputs a spacer line."
+    )
+    assert signature.parameters is not None
+    assert signature.parameters[3].label == (43, 49)
+
+
+def test_innermost_command_is_used_for_nested_call():
+    source = 'set result [SERVICE_spacer_output "*" 20]'
+    tree = CustomParser().parse(source)
+    position = lsp.Position(line=0, character=source.index("20"))
+
+    call = find_active_call(source, tree, position)
+
+    assert call is not None
+    assert call.name == "SERVICE_spacer_output"
+    assert call.active_argument == 1
+
+
+def test_builtin_signature_includes_parameter_documentation():
+    source = "MOM_abort "
+    tree = CustomParser().parse(source)
+
+    result = build_signature_help(
+        source,
+        tree,
+        lsp.Position(line=0, character=len(source)),
+        {},
+        {},
+        standard_items.json_data["MOM_procs"],
+    )
+
+    assert result is not None
+    assert result.active_parameter == 0
+    signature = result.signatures[0]
+    assert signature.label == "MOM_abort <message>"
+    assert signature.parameters is not None
+    assert signature.parameters[0].documentation
+
+
+def test_unknown_command_has_no_signature_help():
+    source = "unknown_custom_command "
+    tree = CustomParser().parse(source)
+
+    result = build_signature_help(
+        source,
+        tree,
+        lsp.Position(line=0, character=len(source)),
+        {},
+        {},
+        standard_items.json_data["MOM_procs"],
+    )
+
+    assert result is None