This monograph explores photokinetics, the study of reaction kinetics under light exposure, addressing its longstanding underdevelopment compared to thermal reaction kinetics. It systematically reviews fundamental concepts, introduces new methods and equations to quantify and predict photochemical and photothermal reaction rates, and provides a standardized framework for laboratory investigations. Covering both monochromatic and polychromatic light effects, the book presents a paradigm shift in the field, making it a valuable reference for students, researchers, and industry professionals. It lays the foundation for the advancement of photokinetics, bridging critical knowledge gaps and setting the stage for future developments. This volume serves as a valuable resource for students, researchers, and academics working with photoreactive systems.