Different applications and uses of light energy have emerged over the last few years in many different fields such as in medicine, material science, energy and biochemistry. New and exciting applications of light-controlled processes have become practical in the diagnosis and treatment of diseases, the preparation and use of functional materials, the storage of solar energy and the control of biological properties. Many of these applications are based on a very simple chemical step: a photoisomerization. The isomerization of a chemical double bond allows for the control with extreme spatial and temporal resolution of complex systems. Nature offers different examples of very complex functions that are initiated by this type of simple chemical. Upon photon absorption, the light energy can be used to induce a chemical (geometrical) change that influences the protein environment that triggers a specific signal or function. Inspired by these amazing and extremely efficient processes, many efforts have been devoted to the modification of natural systems and to the design of new applications, using simple and tunable photoisomerizations. Accordingly, the preparation of photoactive molecular devices based on photoisomerizations and the use of these species in different applications is now a very active scientific field, with profound implications in our everyday lives. In this book, the fundamental aspects of the photoisomerization of many different chemical structures containing C=C, N=N and C=N double bonds is covered. Different experimental and computational tools used to study these processes are discussed and some specific applications of different compounds are presented.