The electromagnetic characterization of layered materials is often performed using reflection-only measurements, and through the years several different techniques have been developed to accomplish this both in the laboratory and in situ. It is shown in this paper that the majority of these methods are variations on a single approach, and that closed-form expressions are available to obtain the permittivity and permeability from the measurements. The approach is applicable to free-space (plane wave) measurements, waveguide (non-TEM) measurements, and transmission line (TEM) measurements, and is based on varying the underlying structure of the material stack to obtain independent measurements. Because the formulation is based on specifying the impedances of adjacent regions, the same formula for permittivity and permeability is valid regardless of the complexity of the stack containing the material under test. Examples of several techniques are given using free-space, waveguide, and transmission-line measurements of both dielectric materials and magnetic absorbing materials.