When a sample of semiconducting material is illuminated, pairs of excess carriers are generated which diffuse through the material according to the density gradients established. Each pair carries an energy approximately equal to the band gap of the material. This energy is deposited where the excess electron recombines with a hole and causes local heating of the lattice. A temperature distribution will therefore be established in the sample which depends on the characteristics of optical absorption and bulk and surface recombination in and on the sample. This establishment of a temperature distribution in a solid by optically excited diffusing and recombining carriers is called the photothermal effect. The paper gives a formulation of the theory governing the photothermal effect, and the case of small temperature elevations in an infinite slab is worked out in detail.