The Elements of Photochemistry was primarily written as a university level text book to introduce the subject to graduate, postgraduate and research students in the field of photophysics, photochemistry and photobiology. 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.