HTTP headers are the conversation before the conversation—how browsers and servers negotiate what to send, how to send it, and what to do with it.
A URL can point at a resource, but it can't negotiate, authenticate, or remember. Request headers give HTTP requests a voice—here's how they speak.
Response headers are how servers answer the questions every request implicitly asks: What is this? How should I handle it? Can I keep a copy? They control everything from caching to security to whether your browser renders HTML or downloads a file.
Without Content-Type headers, browsers can't tell images from spreadsheets, JSON from garbage. Learn how MIME types prevent digital miscommunication.
Cache-Control headers let you decide how long truth stays true—from milliseconds to years. Master the directives that control whether browsers, CDNs, and proxies remember your content or forget it.
Every protected API asks the same question: how do I know you're really you? The Authorization header is your answer—from passwords dressed in Base64 to self-contained tokens that prove themselves.
Security headers are instructions you give browsers because you can't fully trust your own code. Learn how HSTS, CSP, and other headers prevent entire categories of attacks by making the browser enforce what your application might forget.
CORS is the browser playing bouncer—the server already processed your request, but CORS decides whether your JavaScript gets to see what happened.
Custom headers let applications pass their own metadata through HTTP—request IDs that trace a call across fifty services, version numbers that let APIs evolve, feature flags that turn capabilities on and off. Here's how to design them well.
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