This chapter focuses on of level of detail (LOD) for the sizeable topic of terrain visualization. It is of particular importance to realistic flight simulators or terrain-based computer games, as well as to geographic information systems (GISs) and military mission planning applications. A large volume of early work in terrain simplification was done as far back as the late 1970s, and continued through the 1980s and 1990s. this chapter concentrates on the more recent techniques, focusing on real-time view-dependent solutions that are of practical value to game developers or to software engineers concerned with accurate visualization of massive, potentially distributed terrain models. In certain ways terrain is a much easier case to deal with than arbitrary 3D models because the geometry is more constrained, normally consisting of uniform grids of height values. This allows for more specialized and potentially simpler algorithms. Terrain data also bring some added complications making view-dependent LOD techniques of importance for any real-time system. This chapter begins by covering some basic concepts, such as top-down versus bottom-up algorithms; regular, gridded data versus triangulated irregular networks (TINs); various hierarchical structures, such as quadtrees and bintrees; dealing with tears between adjacent patches at different resolutions; out-of-core operation; and streaming over the Web. It focuses on key solutions that provide real-time, view-dependent support, presenting detailed descriptions of each implementation. It closes with some auxiliary discussion of geographic coordinate systems.