The therapeutic effects of macular photocoagulation result from focal heating of the retina and choroid. The magnitude, spatial extent, and duration of temperature increases produced by laser exposures are influenced by light scattering in intraocular and intraretinal transit; light absorption by melanin, hemoglobin, and xanthophyll in target tissues; and beam parameters, such as wavelength, spot size, and exposure duration. Tissue optics and thermodynamics provide a useful guide for selecting new laser systems of potential value in macular photocoagulation, but laser-tissue interactions and subsequent chorioretinal responses are poorly understood, and therapeutic efficacy can be established only by controlled clinical trials.