The role of modeling in designing new treatment protocols and instruments is discussed. A computer program for modeling laser-tissue interaction named Latis is described. Interactions are divided into the processes of laser propagation, thermal effects, material effects, and hydrodynamics. Full coupling of the processes is taken into consideration. Applications in photothermal and photomechanical laser-tissue interactions are briefly discussed. A detailed description is given of a particular application of Latis to study the effects of dynamic optical properties on dosimetry in photothermal therapy. Optical properties are functions of tissue damage, as determined by previous measurements. Results are presented for the time variation of the light distribution and damage within the tissue as the optical properties of the tissue are altered. It is found that proper accounting of dynamical optical properties is important for accurate dosimetry modeling.