When observed as astronomical objects, stellar systems appear projected on the plane of the sky. As a consequence, it is important to set the framework for a correct understanding of the relation between intrinsic dynamics and projected properties. Unfortunately, while it is always possible (at least in principle) to project a model and then compare the results with observational data, the operation of inversion (i.e., the recovery of three-dimensional information starting from projected properties) is generally impossible due to obvious geometric degeneracies. Spherical and ellipsoidal geometries are among the few exceptional cases that will be discussed in some depth in Chapter 13. Here, instead, the reader is provided with some of the general concepts and tools needed for projecting the most important properties of stellar systems on a projection plane.