Responsive web design is a technical methodology where a single HTML codebase adapts its layout, images, and typography based on the visitor's screen width. Instead of serving different URLs for mobile versus desktop (like m.example.com), RWD uses CSS media queries to apply different styles at breakpoints—typically 320px for small phones, 768px for tablets, and 1024px+ for desktops, though modern frameworks now use more granular ranges. The core components are flexible grids (using percentage widths or CSS Grid/Flexbox instead of fixed pixels), fluid images (max-width: 100% so they scale down), and media queries that trigger layout changes. A three-column desktop layout might stack into a single column on mobile, navigation menus collapse into hamburger icons, and touch targets expand to at least 44x44 pixels for usability. Google has treated mobile-friendliness as a ranking signal since 2015 and switched to mobile-first indexing in 2019, meaning it now crawls the mobile version of your site primarily. Non-responsive sites often see 40–60% traffic drops on mobile because users bounce when pinch-zooming is required. From a conversion standpoint, we've measured 15–25% higher form completion rates on properly responsive sites versus scaled-down desktop layouts. At Ottawa SEO, we build all client sites responsive by default using frameworks like Tailwind or Bootstrap, testing across real devices—not just Chrome DevTools. Common mistakes include forgetting to set the viewport meta tag, using fixed-width containers that break on small screens, or loading full-resolution images on mobile (killing page speed). Performance matters: a responsive site that loads 2MB images on a phone isn't truly optimized. RWD isn't a one-time checkbox. You need to test after every content update because a wide table or embedded map can break mobile layout. Tools like Google's Mobile-Friendly Test and real-device testing through BrowserStack catch issues before users do. Responsive design is now table stakes, not a competitive advantage—but poor implementation still tanks rankings and revenue.