Indoors rendering has been a popular technique for game development ever since Wolfenstein 3D by id Software was released. Games taking place inside buildings have become widespread; their visual appeal increasing with each new hardware iteration or engine release. In this chapter, we will explore the building blocks for any indoors engine. Three popular approaches Binary Space Partitions (BSPs), portal rendering, and octree rendering will be explored in full, and some specialized solutions will be covered thereafter. As you will soon discover, indoors games often require quite complex techniques.