We describe three lessons learned about how tissue optics affect the dosimetry of red to near-infrared treatment light during PDT, based on working with Dr. Tayyaba Hasan. Lesson 1-The optical fluence rate near the tissue surface exceeds the delivered irradiance (E). A broad beam penetrates into tissue to a depth (z) as =Eke−;µz, with an attenuation constant µ and a backscatter term k. In tissues, k is typically in the range 3-5, and 1/µ equals , the 1/e optical penetration depth. Lesson 2-Edge losses at the periphery of a uniform treatment beam extend about 3 from the beam edge. If the beam diameter exceeds 6, then there is a central zone of uniform fluence rate in the tissue. Lesson 3-The depth of treatment is linearly proportional to (and the melanin content of pigmented epidermis in skin) while proportional to the logarithm of all other factors, such as irradiance, exposure time, or the photosensitizer properties (concentration, extinction coefficient, quantum yield for oxidizing species). The lessons illustrate how tissue optics play a dominant role in specifying the treatment zone during PDT.