It is now well established that all living systems emit a weak but permanent photon flux in the visible and ultraviolet range. This biophoton emission is correlated with many, if not all, biological and physiological functions. There are indications of a hitherto-overlooked information channel within the living system. Biophotons may trigger chemical reactivity in cells, growth control, differentiation and intercellular communication, i.e. biological rhythms. The basic experimental and theoretical framework as well as the technical problems and the wide field of applications in the biotechnical, biomedical engineering, engineering, medicine, pharmacology, environmental science and basic science fields are presented in this book. To promote the dialog and mutual penetration between biophoton research and photon technology is one of the important goals for the International Conference on Biophotons & Biophotonics 2003, and is developed and presented in Biophotonics: optical science and engineering in the 21st century.