Photobiology deals with the interaction of light with biological matter. Hence, it forms the core of biophotonics which utilizes interactions of light with biological specimens. Here we discuss the interactions of various molecular, cellular and tissue components with light. The content is mostly taken from Chapter 6 of this author's monograph on biophotonics (Prasad, 2003). The various light-induced radiative and non-radiative processes are described, along with a discussion of the various photochemical processes. Photochemistry in cells and tissues can also be initiated by externally added exogenous substances, often called photosensitizers, which form the basis for photodynamic therapy. The various types of scattering processes occurring in a tissue as described here, together with light absorption, determine the penetration of light of a given wavelength into a particular type of tissue. Methods of measurement of optical reflection, absorption and scattering properties of a tissue are introduced. Some important manifestations of non-radiative processes in a tissue, used for a number of biophotonics applications such as laser tissue engineering and laser microdissection are thermal, photoablation, plasma- induced ablation and photodisruption. These processes are defined. An emerging area of biophotonics is in vivo imaging and spectroscopy for optical diagnosis. This topic is covered, along with the various methods of light delivery for in vivo photoexcitation. Another exciting in vivo biophotonics area is that of optical biopsy to detect the early stages of cancer. This topic is covered as well. Understanding of structure and functions at the single biomolecule and bioassembly levels is a major thrust of molecular and structural biology. This topic is also covered here. The use of novel optical techniques allows one to probe processes at the single molecule level.