CSS Flexbox and Grid are the two modern layout systems that fundamentally transformed how web developers structure page layouts, replacing decades of workarounds involving floats, inline-block, table display, and absolute positioning. Understanding when to use each system, and how they complement each other, is essential for efficient modern web development. The CSS box model underpins both systems: every element generates a rectangular box with content, padding, border, and margin areas, and both Flexbox and Grid control how these boxes are sized and positioned within their container.

Flexbox (Flexible Box Layout), standardized in 2012 and reaching full browser support by 2015, is designed for one-dimensional layout along either a row or a column. A flex container distributes space among its flex items according to flexible sizing rules. The justify-content property controls distribution along the main axis (the direction items flow), while align-items controls positioning along the cross axis (perpendicular to the flow). Flex items can grow to fill available space (flex-grow), shrink to avoid overflow (flex-shrink), and establish a base size before growing or shrinking (flex-basis). The flex-wrap property allows items to wrap to multiple lines when they exceed the container width, and align-content controls the spacing between wrapped lines.

CSS Grid Layout, standardized in 2017, provides two-dimensional layout control over both rows and columns simultaneously. A grid container defines explicit tracks using grid-template-columns and grid-template-rows, with the fr (fractional) unit distributing available space proportionally. The repeat() function creates repeating track patterns, and minmax() sets minimum and maximum track sizes. Grid items can span multiple tracks using grid-column and grid-row properties, and named grid areas (grid-template-areas) allow layouts to be defined using ASCII-art-like visual templates. The auto-fill and auto-fit keywords with repeat() enable responsive grids that automatically adjust column count based on available space, without media queries.

The practical distinction between Flexbox and Grid determines which to use in each situation. Flexbox excels when you need content to determine the layout: navigation bars where items have natural sizes, card rows that distribute available space, centering content both vertically and horizontally, and any scenario where items should flow and wrap naturally. Grid excels when you need the layout to determine the content placement: page-level layouts with headers, sidebars, and content areas, image galleries with consistent sizing, dashboard layouts with cards of varying spans, and any design where you want to define the structure first and place content into it. The two systems work together naturally: a Grid container can have Flexbox-based items, and Flex containers can be Grid items, enabling nested layouts that leverage the strengths of both systems.