Many interdependent variables are involved in the processes of manufacture, exposure, and processing of silver halide photosensitive materials. Understanding of the conditions and mechanisms for emulsion grain growth, chemical sensitization, spectral sensitization, latent image formation, and development is facilitated by discussion based upon a number of physico-chemical concepts which have evolved from the earliest days of photographic experience. There has been much controversy over the relevance and validity of alternative hypotheses and theories which have been proposed, adopted, and then vigorously defended, some after they have outlived their usefulness. The purpose of this paper is to trace the historical development of basic concepts of photographic sensitivity with emphasis on principles of physics and chemistry and relevance to emulsion technology.