Considerable time and effort has been devoted to predictions of the extent of thermal damage owing to laser photocoagulations. To establish a suitable radiation dose requires a complete understanding of the optical-thermal response of tissue to laser radiation and an accurate assessment of the optical and thermal properties of tissue. This is a formidable and mostly unsolved problem. Even the best predictions and algorithms are no better than the accuracy of measurement of parameters used in the governing equations. One problem is the tissue optical properties. Typically these properties are determined from in vitro samples. From the wide range of values of absorption and scattering coefficients reported in the literaturel, it is difficult to judge the average optical properties of a particular in vivo tissue, much less extract values for a particular region that will be radiated. One solution eliminates dosimetry uncertainty by monitoring the progression of photocoagulation during laser radiation and stopping irradiation when a specified endpoint has been reached.