CSS preprocessors extend CSS with variables, nesting, and functions. Sass is the most mature and widely adopted preprocessor. Variables values like colors and font sizes. Nesting mirrors HTML structure for cleaner stylesheets. Mixins create reusable style blocks with arguments. Functions compute values dynamically. Partials and @import organize styles into modules. Extend/inheritance shares rule sets between selectors. Control directives like @if and @for enable logic in styles. Sass uses SCSS syntax, compatible with CSS, or indented syntax. Less is similar to Sass but uses JavaScript-based compilation. Less has broader framework integration like Bootstrap. Stylus offers minimal syntax with optional colons and semicolons. Stylus supports both indented and CSS-like syntax. All three support variables, nesting, and mixins. Sass has the largest ecosystem and community resources. Autoprefixer handles vendor prefixes regardless of preprocessor choice. PostCSS processes CSS with plugins, replacing traditional preprocessors. PostCSS combined with Sass provides maximum flexibility. Choose Sass for stability and community support.