The book presents concepts, facts and theories of photophysical and photochemical phenomena in a complete and lucid manner. Photochemical reactions occur in the electronically excited states which, because of their multiplicity provide many pathways for the reaction to follow. The creation and detection of excited states and their physical and chemical properties have been discussed from their fundamentals. An attempt has been made to emphasize the importance of spectroscopy and group theory in the understanding of photochemistry. The book stresses the importance of the Frank-Condon principle in radiative and nonradiative transfer of energy. It also emphasizes the nature and role of singlet oxygen in photo-peroxidation, atmospheric photochemistry and environmental photochemistry.