AbstractLasers provide a means of delivering high intensity light to small well‐defined areas under precise control. The biological response depends on the light wavelength and intensity and the absorption characteristics of the target organ. The most important effects are thermal and include tissue vaporization, necrosis with later sloughing, and necrosis stimulating an inflammatory response which may lead to local fibrosis. The Carbon Dioxide Laser can cut or vaporize neoplastic tissue in areas accessible to rigid endoscopy, but the more penetrating Nd YAG and Argon laser beams can be transmitted via flexible fibers and have greater potential for destroying larger tumors without unacceptable damage to surrounding areas. More selective tumor phototherapy is possible in some organs by sensitization with HpD (hematoporphyrin derivative) and subsequent treatment with a dye laser. This effect is non‐thermal and depends on the production of singlet oxygen by activated HpD. The precision possible for local treatment of solid tumors with lasers is greater than for almost any other techniques, but careful quantitative studies are needed to establish the appropriate treatment parameters in any particular situation.