diff --git a/omicron/ssg/cli.py b/omicron/ssg/cli.py
index cee1fcf..069dd6a 100644
--- a/omicron/ssg/cli.py
+++ b/omicron/ssg/cli.py
@@ -14,7 +14,7 @@ def setup_logging(verbose: bool = False, log_file: str | None = None) -> None:
     logger.addHandler(console)
 
 
-def main():
+def main() -> None:
     setup_logging(verbose=True)
     site = Site(Path("."))
     site.build()
diff --git a/omicron/ssg/content.py b/omicron/ssg/content.py
index 8e47f88..f10b550 100644
--- a/omicron/ssg/content.py
+++ b/omicron/ssg/content.py
@@ -3,9 +3,9 @@ import re
 from abc import ABC, abstractmethod
 from pathlib import Path
 from datetime import date
-import typing
+from typing import TYPE_CHECKING, Any, cast
 
-if typing.TYPE_CHECKING:
+if TYPE_CHECKING:
     from omicron.ssg.site import Site
 
 import yaml
@@ -34,19 +34,26 @@ def _highlight_code(code: str, lang: str, attrs: str) -> str:
     return pygments_highlight(code, lexer, HtmlFormatter())
 
 
-def read_frontmatter(path) -> dict:
+def read_frontmatter(path: Path) -> dict[str, Any]:
     lines = []
     with path.open("r", encoding="utf-8") as f:
         if f.readline() != "---\n":
             raise ContentError(f"Frontmatter opening delimiter not found for {path}")
         while (line := f.readline()) != "---\n":
             if line == "":
-                raise ContentError(f"Frontmatter closing delimiter not found for {path}")
+                raise ContentError(
+                    f"Frontmatter closing delimiter not found for {path}"
+                )
             lines.append(line)
-    return yaml.safe_load("".join(lines))
+    data = yaml.safe_load("".join(lines))
+    if not isinstance(data, dict) or not all(isinstance(k, str) for k in data):
+        raise ContentError(
+            f"Frontmatter must be a YAML mapping with string keys in {path}"
+        )
+    return cast(dict[str, Any], data)
 
 
-def create_content(path) -> Content:
+def create_content(path: Path) -> Content:
     if path.suffix in FRONTMATTER_CONTENT:
         meta = read_frontmatter(path)
     elif path.suffix == ".yml":
@@ -93,7 +100,9 @@ class Content(ABC):
             f.readline()  # opening ---
             while (line := f.readline()) != "---\n":
                 if line == "":
-                    raise ContentError(f"Frontmatter closing delimiter not found for {self.source}")
+                    raise ContentError(
+                        f"Frontmatter closing delimiter not found for {self.source}"
+                    )
             return f.read()
 
     @abstractmethod
@@ -102,7 +111,7 @@ class Content(ABC):
 
 
 class Article(Content):
-    def __init__(self, path: Path, meta: dict):
+    def __init__(self, path: Path, meta: dict[str, Any]):
         if "tags" not in meta:
             log.warning("no tags for article in '%s'", path)
         super().__init__(
diff --git a/omicron/ssg/site.py b/omicron/ssg/site.py
index 5f6c81b..bbaea42 100644
--- a/omicron/ssg/site.py
+++ b/omicron/ssg/site.py
@@ -1,5 +1,5 @@
 import logging
-from typing import Any
+from typing import Any, cast
 
 from omicron.ssg.content import (
     Content,
@@ -39,11 +39,13 @@ class Site:
         log.debug("Reading config file %s", config_path)
         with open(config_path, "r") as f:
             config = yaml.safe_load(f)
+        if not isinstance(config, dict) or not all(isinstance(k, str) for k in config):
+            raise ConfigError("config file must be a YAML mapping with string keys")
         if "name" not in config:
             raise ConfigError("name value missing from config")
         if "template" not in config:
             raise ConfigError("template value missing from config")
-        return config
+        return cast(dict[str, Any], config)
 
     @staticmethod
     def resolve_template_path(template: str) -> Path:
@@ -84,7 +86,7 @@ class Site:
                 self.content.append(content)
         log.info("Discovered %d content items", len(self.content))
 
-    def build(self):
+    def build(self) -> None:
         self.discover()
         self.output_path.mkdir(parents=True, exist_ok=True)
         (self.output_path / "pygments.css").write_text(
diff --git a/pyproject.toml b/pyproject.toml
index d2444d1..9410503 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,9 +13,19 @@ dependencies = [
     "PyYAML",
 ]
 
+[project.optional-dependencies]
+dev = [
+    "mypy",
+    "types-PyYAML",
+    "types-Pygments",
+]
+
 [project.scripts]
 ossg = "omicron.ssg.cli:main"
 
+[tool.mypy]
+strict = true
+
 [tool.setuptools.packages.find]
 where = ["."]
 include = ["omicron.ssg*"]