This document discusses responsive web design. It begins by explaining that current grid/layout thinking is based on pages, but with responsive design there is no page. It then discusses how responsive design works by using fluid grids, flexible images and media, and media queries to adapt the layout based on screen size. It provides examples of how to implement a fluid grid and make images flexible. Finally, it gives some tips for responsive design such as using relative sizing and media queries to define breakpoints for different screen widths.