The properties of excess carriers in semiconductors are some of the more interesting aspects of microelectronics. The role these excess carriers play is important from the simplest transistor to the most elegant photo-voltaic device (and conversely, from the simplest photo-conductive device to the most elegant bipolar transistor). In this chapter, the role of these optically created, non-equilibrium carriers is discussed. While the approach depends nominally upon the Boltzmann equation, it is more usual to work with the continuity equation, incorporating current primarily through the relaxation-time approximation. The aspects of diffusion of minority carriers will be treated first, as it is of primary importance to Eater discussions of optically generated carriers and their transport. The discussion then turns to optical absorption and recombination. Finally, a discussion of ultra-fast laser excitation of semiconductors is presented, with simulations using the ensemble Monte Carlo technique.