Tailwind CSS handles responsive design through breakpoint prefixes you add directly to utility classes. Write your base styles for mobile (no prefix), then use sm: for 640px+, md: for 768px+, lg: for 1024px+, xl: for 1280px+, and 2xl: for 1536px+. For example, "text-sm md:text-base lg:text-lg" starts small on mobile and increases at tablet and desktop widths. The mobile-first approach means unprefixed classes apply to all screen sizes unless overridden. A common pattern is "w-full md:w-1/2 lg:w-1/3" for a full-width mobile card that becomes half-width on tablets and one-third on desktops. Grid layouts work similarly: "grid-cols-1 md:grid-cols-2 lg:grid-cols-3" stacks everything on mobile, two columns on tablets, three on desktops. Hiding and showing elements uses the same logic. "hidden lg:block" hides something on mobile and tablets but shows it on large screens. Padding and margin scale well with "px-4 md:px-8 lg:px-16" for increasing horizontal spacing as viewport grows. At Ottawa SEO, we typically start every component mobile-first in Tailwind, test at 375px width (iPhone SE baseline), then add md: breakpoint adjustments around 768px for tablets, and lg: refinements at 1024px for desktops. We rarely use xl: or 2xl: unless the client specifically needs ultra-wide monitor optimization. Watch out for over-nesting breakpoints in the same class string. If you have seven breakpoint variants on one element, you probably need to extract that into a custom component or use Tailwind's @apply directive in your CSS. Also remember that breakpoints are min-width, not max-width, so styles cascade upward. A md:text-xl will stay xl at lg: and above unless you explicitly override it. Custom breakpoints go in your tailwind.config.js theme.screens object if the defaults don't match your design system, but the five standard breakpoints cover 95% of real-world responsive needs.