This book focuses on physics and associated mathematical relationships. Basic principles are first presented, and then practical applications and results are detailed. The intent here is to present a working view of physical phase mechanics, then followed by application to decompression theory in diving. It is directed toward the diver and reader with a basic understanding of decompression. Basically the mechanics of tissue gas exchange, bubbles and nucleation, supersaturation, perfusion and diffusion, and related mechanisms are discussed. The targeted audience is especially the doctor or physiologist, physicist, chemist, mathematician, engineer or biologist by training and also the commercial diver, technical diver, instructor, hyperbaric technician, underwater researcher, looking for greater detail.