diff --git a/i18n_key_sync/validate.py b/i18n_key_sync/validate.py
new file mode 100644
index 0000000..905e7d5
--- /dev/null
+++ b/i18n_key_sync/validate.py
@@ -0,0 +1,122 @@
+"""Validation of i18n keys against locale files."""
+
+from dataclasses import dataclass, field
+from typing import Dict, Set
+
+
+@dataclass
+class ValidationResult:
+    """Result of validating i18n keys against locale files."""
+
+    missing_keys: Set[str] = field(default_factory=set)
+    unused_keys: Set[str] = field(default_factory=set)
+    matching_keys: Set[str] = field(default_factory=set)
+    extra_keys_per_locale: Dict[str, Set[str]] = field(default_factory=dict)
+
+    @property
+    def total_missing(self) -> int:
+        """Total count of missing keys."""
+        return len(self.missing_keys)
+
+    @property
+    def total_unused(self) -> int:
+        """Total count of unused keys."""
+        return len(self.unused_keys)
+
+    @property
+    def total_matching(self) -> int:
+        """Total count of matching keys."""
+        return len(self.matching_keys)
+
+    @property
+    def is_valid(self) -> bool:
+        """Check if there are no missing or unused keys."""
+        return len(self.missing_keys) == 0 and len(self.unused_keys) == 0
+
+
+class Validator:
+    """Validate extracted i18n keys against locale files."""
+
+    def __init__(self):
+        """Initialize the validator."""
+
+    def validate(
+        self,
+        extracted_keys: Set[str],
+        locale_keys: Dict[str, Set[str]],
+    ) -> ValidationResult:
+        """Validate extracted keys against locale keys.
+
+        Args:
+            extracted_keys: Set of keys extracted from source code.
+            locale_keys: Dictionary mapping locale names to their key sets.
+
+        Returns:
+            ValidationResult with missing, unused, and matching keys.
+
+        """
+        if not locale_keys:
+            return ValidationResult(
+                missing_keys=extracted_keys,
+                unused_keys=set(),
+                matching_keys=set(),
+            )
+
+        result = ValidationResult()
+
+        for locale, keys in locale_keys.items():
+            locale_missing = extracted_keys - keys
+            locale_unused = keys - extracted_keys
+            locale_matching = keys & extracted_keys
+
+            result.missing_keys.update(locale_missing)
+            result.unused_keys.update(locale_unused)
+            result.matching_keys.update(locale_matching)
+            result.extra_keys_per_locale[locale] = locale_unused
+
+        return result
+
+    def validate_for_locale(
+        self,
+        extracted_keys: Set[str],
+        locale_keys: Set[str],
+        locale_name: str = "",
+    ) -> ValidationResult:
+        """Validate keys against a single locale.
+
+        Args:
+            extracted_keys: Set of keys extracted from source code.
+            locale_keys: Set of keys in the locale file.
+            locale_name: Name of the locale (for reporting).
+
+        Returns:
+            ValidationResult for the single locale.
+
+        """
+        missing_keys = extracted_keys - locale_keys
+        unused_keys = locale_keys - extracted_keys
+        matching_keys = locale_keys & extracted_keys
+
+        return ValidationResult(
+            missing_keys=missing_keys,
+            unused_keys=unused_keys,
+            matching_keys=matching_keys,
+        )
+
+    def get_summary(self, result: ValidationResult) -> Dict[str, int]:
+        """Get a summary of the validation result.
+
+        Args:
+            result: ValidationResult to summarize.
+
+        Returns:
+            Dictionary with summary statistics.
+
+        """
+        return {
+            "missing_keys": len(result.missing_keys),
+            "unused_keys": len(result.unused_keys),
+            "matching_keys": len(result.matching_keys),
+            "total_keys_in_code": len(result.missing_keys) + len(result.matching_keys),
+            "total_keys_in_locale": len(result.unused_keys) + len(result.matching_keys),
+        }