While the creation of multichannel scores and ambiences has become increasingly common, perhaps the most important element of 5.1 implementation is the use of 3D-positioning. After covering some of the basic concepts of 3D, including distance-based attenuation and panning, the topics move into newer techniques that allow for more immersive 3D positioning: distance-based filtering, directional sound sources, the positioning of multichannel source material, and reverberation simulation. Prioritization and culling techniques for positioned sound sources are also discussed.