Optical components (such as mirrors, lenses, and prisms), mounts, brackets, and housings are a few examples of bodies that make up an optical system or instrument. Each of these bodies must be mounted to or within the structure of the instrument. Common mounting methods include bolts, clamps, retaining rings, and adhesives. Usually, these are sufficient to guarantee that the various bodies stay in place when subjected to operational and environmental loads. However, for optical systems, these often are not adequate because of stringent requirements for positional accuracy, stability, and freedom from mechanical strain. Then, it becomes evident that much skill and craftsmanship are needed to realize design objectives. Ways to do so are described throughout this book. This chapter concentrates on the basic principles for mounting various bodies. Each is kinematically mounted and all of their six degrees of freedom (DOFs) are independently constrained once or intentionally left free for adjustment purposes. These bodies are assumed to be rigid and infinitely stiff. Overconstraints usually result in loads on the components that will tend to deform them, thereby degrading the instrument’s performance.