Optics for laser irradiation of tissue is best described by examining the response of a target within tissue to light. Suppose tissue (e.g. skin) has a chromophore, (e.g. a melanocyte) somewhere inside the tissue at coordinate with respect to some frame of reference (Fig. 3.1). What is the rate of heat that is generated in the chromophore when the tissue is irradiated at some wavelength with constant power P over the laser beam radius w L ? The key question that needs to be answered is: how many photons per second will reach the chromophore and be absorbed. Tissue optics should provide the answer to this question.