The optical control of molecular systems and related physical properties (electronic state, magnetic, optical, etc.) by laser irradiation has gained tremendous interest in fields such as femtochemistry of molecules in gas phase or solution, and is now picking up momentum within the emerging field of photoinduced phase transitions of molecules in solids. Light-induced changes of molecular systems involve subtle coupling between the electronic and structural degrees of freedom, which are essential to stabilize the photoexcited state, different in nature from the stable state. Therefore the new experimental field of photocrystallography plays a key role in probing excited state structures. By playing on different physical parameters and developing the techniques and analysis, one can investigate new out-of-equilibrium physics through light-driven cooperative dynamics and transformations in materials, or follow a chemical reaction in real time. This chapter reviews different aspects of the use of several techniques to study structural aspects of excited states in solution or in the solid state, in long-lived states, or short-lived transients.