A scalable extension to the H.264/AVC video coding standard has been developed within the joint video team (JVT), a joint organization of the ITU-T video coding group (VCEG) and the ISO/IEC moving picture experts group (MPEG). The extension allows multiple resolutions of an image sequence to be contained in a single bit stream. In this paper, we introduce the spatially scalable extension within the resulting scalable video coding standard. The high-level design is described and individual coding tools are explained. Additionally, encoder issues are identified. Finally, the performance of the design is reported.