Responsive web design is the practice of creating websites that automatically adapt their layout, images, and typography to suit any screen size or device. Emerging from Ethan Marcotte's foundational work in 2010, responsive design has evolved into the standard approach for building flexible, device-agnostic web experiences. The core technique combines fluid grids, flexible images, and media queries to deliver content optimized for contexts ranging from smartphones to ultrawide monitors. Modern responsive design extends beyond simple breakpoints: it embraces container queries for component-level responsiveness, CSS functions like clamp() for fluid scaling, and accessibility-first patterns like touch target sizing. A critical mindset shift is mobile-first design, where you build for the smallest screen first then progressively enhance for larger displays—ensuring performance and usability where constraints matter most.