Progress in laser technology and improved understanding of laser-tissue interactions has spawned a new generation of laser applications for ophthalmic surgery. Unfortunately, the mechanisms and terminology of these new applications can be confusing, and it may seem to the more casual observer that new laser terminology is restricted only by the number of terms to which the prefix photo may be attached. In reality, however, there are only a limited number of ways in which laser light is currently known to interact with ocular tissues, and only limited knowledge of these interactions is needed to understand where each new laser application fits into the growing photoforest of new surgical techniques. Since a laser beam is directional and has little divergence, it can be focused into a small spot size of high irradiance (irradiance is delivered divided by area exposed, often less properly termed power density). Laser applications in surgery generally require high irradiances, whether the lasers are used to produce: (1) thermal effects, such as retinal photocoagulation with an argon laser, (2) ionizing effects, such as photodisruption with a neodymium (Nd):YAG laser, or (3) photochemical effects, such as melanoma photoradiation with an organic dye laser. Current laser applications in ophthalmic surgery may be divided into one of these three categories, as shown in Figure 1. Thermal effects occur when a laser beam is used to heat tissues, such as the trabecular meshwork or the retinal pigment epithelium, to a temperature high enough to produce a therapeutic tissue alteration. In a thermal process, photons in the laser beam are captured by lightabsorbing pigments in the target tissue, and energy from the photons increases the average motion of molecules in the tissue. The increase in microscopic molecular motion is equivalent macroscopically to an increase in the temperature of the target tissue. Since microscopic molecular collisions spread this increased motion to surrounding molecules, temperature also increases in areas