This review presents theoretical and experimental aspects of (e,2e) spectroscopy specific to the study of crystalline and amorphous solids. The cross section for (e,2e) scattering is proportional to the spectral momentum density of the ejected electron under certain approximations. The theoretical framework for interpreting (e,2e) measurements is summarized here and general properties of the spectral momentum density of solids are discussed. Different designs of the (e,2e) spectrometer are described and sample preparation techniques are briefly reviewed. A summary of recent (e,2e) experiments on the electronic structure of valence electrons in graphite, ion sputtered and evaporated amorphous carbon, and aluminum-aluminum oxide films is given. The review concludes with a discussion of an "ideal" (e,2e) spectrometer for studying bulk properties of solids.