diff --git a/src/depcheck/models.py b/src/depcheck/models.py
new file mode 100644
index 0000000..8f69477
--- /dev/null
+++ b/src/depcheck/models.py
@@ -0,0 +1,91 @@
+"""Data models for depcheck."""
+
+from dataclasses import dataclass, field
+from enum import Enum
+from typing import Optional
+
+
+class Severity(str, Enum):
+    """Severity levels for vulnerabilities and updates."""
+
+    CRITICAL = "critical"
+    HIGH = "high"
+    MEDIUM = "medium"
+    LOW = "low"
+    INFO = "info"
+
+
+class PackageManager(str, Enum):
+    """Supported package managers."""
+
+    NPM = "npm"
+    PIP = "pip"
+    GO = "go"
+    CARGO = "cargo"
+
+
+@dataclass
+class Dependency:
+    """Represents a single dependency."""
+
+    name: str
+    current_version: str
+    package_manager: PackageManager
+    latest_version: Optional[str] = None
+    is_outdated: bool = False
+    vulnerabilities: list = field(default_factory=list)
+    category: str = "dependencies"
+    source_file: Optional[str] = None
+
+    def __hash__(self):
+        return hash((self.name, self.package_manager.value, self.current_version))
+
+    def __eq__(self, other):
+        if not isinstance(other, Dependency):
+            return False
+        return (
+            self.name == other.name
+            and self.package_manager == other.package_manager
+            and self.current_version == other.current_version
+        )
+
+
+@dataclass
+class Vulnerability:
+    """Represents a security vulnerability."""
+
+    cve_id: str
+    severity: Severity
+    description: str
+    affected_versions: str
+    fixed_version: Optional[str] = None
+    references: list[str] = field(default_factory=list)
+
+
+@dataclass
+class ScanResult:
+    """Result of a dependency scan."""
+
+    dependencies: list[Dependency] = field(default_factory=list)
+    vulnerabilities: list[tuple[Dependency, Vulnerability]] = field(default_factory=list)
+    scan_errors: list[str] = field(default_factory=list)
+    package_manager: Optional[PackageManager] = None
+    source_file: Optional[str] = None
+
+    @property
+    def outdated_count(self) -> int:
+        return sum(1 for d in self.dependencies if d.is_outdated)
+
+    @property
+    def vulnerable_count(self) -> int:
+        return len(self.vulnerabilities)
+
+    def get_highest_severity(self) -> Optional[Severity]:
+        if not self.vulnerabilities:
+            return None
+        severity_order = [Severity.CRITICAL, Severity.HIGH, Severity.MEDIUM, Severity.LOW]
+        for severity in severity_order:
+            for dep, vuln in self.vulnerabilities:
+                if vuln.severity == severity:
+                    return severity
+        return None