So far, the energy necessary for a change to take place had to be brought in by the reactants. In this chapter the required energy is provided by light. Often, light allows us to promote the system to an electronically excited state where the potential and hence the dynamics are different from those of the ground electronic state. But light-induced chemistry or “photochemistry” is not only a new way of driving chemical reactions. Photochemistry offers a degree of control – a selectivity – that is unique in its potential and variety. Our ability to tailor light toward specific applications is part of the secret. This control ranges from a simple requirement such as wavelength or polarization selection that allows us to access a particular excited state to more complex tasks such as manipulations that produce short light pulses that can freeze the motion and to pulse shaping in frequency and time for the purpose of guiding nuclear motions. Another important aspect is the ingenuity of scientists in matching up light and matter. In particular we have learned to take advantage of the changing character of the intramolecular dynamics of molecules as the energy increases. At low levels of excitation polyatomic molecules offer us a great individuality through their spectroscopic fingerprints, whereas at higher levels of excitation they have a quasi-continuum of vibrational states and can be made to take up lots of energy like a good heat sink. This saga is nowhere near to a closed chapter.