Many books cover the determination of rate constants under different experimental conditions and different chemical composition of the reaction mixture in their formal treatment of thermal kinetics. However, most textbooks are limited to simple mechanisms. In contrast, analogous treatment of photochemical reactions is limited to the publication of special reactions and investigations. Therefore, this book is aimed at providing an overall description of formal photokinetics covering a wider scope than the usual books on kinetics. This volume attempts to provide a concise treatment of both thermo- and photochemical reactions by means of generalised differential equations, their set-up in matrix notation, and their solution by a formalism using numerical integration. At a first glance, this approach might be surprising. However, apart from the argument that the didactics of thermal reactions are easier to handle than those of kinetics, the book provides additional reasons in support of this approach. Therefore, the formalism derived allows the evaluation of photochemical reactions, which are superimposed thermal reactions taking into account that the amount of light absorbed varies during the reaction. Because of this, any approximation, either by using total absorbance or negligible absorbance, will cause considerable errors even for simple reactions. The approach chosen to transform the axis of the radiation time into a new variable that includes the photokinetic factor proves that formal kinetics can be applied to thermal and photochemical reactions as well, and even allows the handling of solutions that cannot be homogenised or solid samples in which the concentration varies locally. By using this approach to introduce partial photochemical quantum yields even complex mechanisms can be determined quantitatively. A large number of examples for different mechanisms and an introduction to many spectroscopic and chromatographic methods suitable for photokinetic analyses are provided to enable the reader to carry out a step-by-step evaluation of his own measurements. To reduce the number of formula in some chapters, an appendix has been included which contains a detailed description of the calculus of some essential examples. For the convenience of the reader, the following has been included: a large number of examples describing the use of formula; a detailed description of the procedure for applying photokinetics to complex consecutive photoreactions; an Internet address where the reader can find a tutorial for this procedure; and a simple macro to help in programming his own evaluation procedure.