Optical transitions in condensed matter are electronic transitions between an initial and a final electron state induced by electromagnetic radiation. This thesis focuses on the mechanisms of those transitions in the case of semiconductor materials. The optical transitions in semiconductors are determined entirely by their electronic structure. Thus, electronic energy bands were introduced as a consequence of the effect of periodic crystal lattice on free electrons. The notion of a valence band which is associated with the initial electron state and that of a conduction band which is associated with the final electron state was described, together with the forbidden energy gap. In addition, a joint density of states was introduced, which describes the number of filled states in the valence band and the number of empty states in the conduction band. Also, the existance of points in k-space (called critical points) in which the desnity of states diverged was brought to attention. A semiclassical theory of interaction of the electrons with the electromagnetic radiation was introduced, in order to describe the optical transitions in semiconductors. The term ”semiclassial” arises from the fact that the radiation was treated classically, while the electrons were treated quantum-mechanically. The semiclassical description yielded important results that were used in derivation of the absorption coefficient that depended on photon energy. This was done both in the case of semiconductors with a direct and an indirect absorption edges. Finally, the discussion of optical transitions and the absorption coefficient was expanded to the case of a nonzero absorption coefficient in the cases of both crystalline and amorphous semiconductors.