This course offers an introduction to photochemistry for students with a good knowledge of quantum mechanics. A minimal background in physical and theoretical chemistry or molecular physics is beneficial. The focus is on general and fundamental concepts and particularly on excited state dynamics. The goal is to understand the various photochemical processes that occur after light absorption in molecules, nanostructures and at interfaces. The theoretical tools to interpret both traditional steady state experiments and recent ultrafast laser spectroscopy are provided. Modern quantum chemical approaches to compute excited state energies and potential energy surfaces are briefly discussed. In the last part, case studies illustrate the photochemical concepts applied to molecules, as well as to photovoltaic and energy conversion materials. The course contains practical sessions in the computer lab, which require very basic knowledge of Linux and Python.