The study of transport phenomena in solids is reviewed, with emphasis on the successes of various theoretical treatments in explaining experimental studies. Basic emphasis is given to the use of the Boltzmann equation, and its application to studies of equilibrium electronic transport (mobility, galvanomagnetic effects), including the effects of carrier‐carrier scattering. A discussion of the progress in the study of nonequilibrium electronic transport (a solid in the presence of a strong applied electric field) is also presented. The extensive experimental effort involving instabilities in semiconductors has provided a fertile testing ground for theoretical studies, and has led to considerable progress in solutions of the nonequilibrium transport equations in the past several years. It is suggested that new phenomena are still providing challenges for theoretical work, as exemplified by the recent studies of electron‐hole “liquid” drops (phase transition) in semiconductors.