What are the most important HTTP request and response headers in REST APIs?

Key request headers: Content-Type declares the format of the request body (e.g., application/json). Accept declares the response format the client prefers. Authorization carries authentication credentials (e.g., Bearer {token}). Accept-Language specifies the preferred language for the response. Key response headers: Content-Type declares the response body format. Location provides the URL of a newly created resource (201 responses). X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset communicate rate limit status. ETag is a hash of the resource for cache validation. Cache-Control instructs caches how long to store the response. WWW-Authenticate indicates the authentication scheme required (on 401 responses).