APIs are critical infrastructure for modern applications, but they're also attack targets. Exposed APIs can leak sensitive data or enable unauthorized actions. Implement API authentication using OAuth 2.0 or similar standards. Rate limiting prevents brute force attacks. Input validation and output encoding prevent injection attacks. Monitor API usage for suspicious patterns. Log all API access for audit trails. Regular security testing should include API security assessments. API security isn't separate from application security. It's integrated into your overall security strategy, protecting the connections that power modern business.