This chapter and the next are the core of this book’s explanation of the 3D-printing process. 3D printers cannot use a computer model until that model is changed into commands to move around an extruder (see Chapter 1) and control when that extruder is and is not putting down material. Because the types of 3D printers controlled by MatterControl lay up a layer at a time of some material, the first part of the software workflow (described in this chapter) cuts the model into layers, normally referred to as slicing. 3D printing is easier than machining, but a lot harder than (2D) consumer printing. We have found cooking to be an apt metaphor, and will expand on that shortly.