A 3D model can be defined inside the modeling software either as a solid or as a hollow body (in other words, it is made up of external surfaces). As mentioned in the introduction to this chapter, AutoCAD offers the option to create the 3D model either way. In this section, surface modeling will be discussed, the two surface types available in AutoCAD (planar and network) will be introduced, and the creation and modification commands will be explored in detail.