When building a responsive website with modern CSS, developers primarily rely on two powerful layout modules: Flexbox (Flexible Box Layout Module) and CSS Grid. While both are essential, they serve distinct purposes. Flexbox is fundamentally a one-dimensional system, excelling at aligning and distributing space among items in a single row or a single column. It’s perfect for component-level layouts, such as navigation bars or form controls. In contrast, CSS Grid is a two-dimensional system, designed for complex, page-level layouts where precise control over rows and columns simultaneously is required. The key is knowing when to use which: Flexbox for content distribution inside components, and Grid for overall page structure.