API design is the process of creating well-structured interfaces for programmatic access to system capabilities, centered on the REST (Representational State Transfer) architectural style—a set of constraints that shape how web services interact over HTTP. Good API design balances discoverability, consistency, and performance, enabling developers to predict behavior, integrate quickly, and build reliably at scale. The key insight: RESTful design is not just about using HTTP verbs—it's about treating resources as first-class citizens, embracing statelessness, and leveraging hypermedia to guide clients through application state transitions. Master these principles, and you'll craft APIs that evolve gracefully, perform efficiently, and delight every developer who uses them.