Activity in the photochemistry of a-diketones has continued unabated in the past decade. In addition to special attention to absorption and emission spectra, photoelectron spectroscopy has been applied widely. Areas of recent emphasis include (1) cyclobutene- and (2) cyclobutanediones, (3) bridged cyclohexenediones, and (4) reactions in the presence of oxygen, particularly epoxidation of olefins. The more venerable aspects such as inter- and intramolecular hydrogen atom abstraction reactions and additions to multiple bonds continue to receive attention. A considerable number of examples of synthetic applications have accumulated in recent years. In parallel, mechanistic understanding has broadened considerably.