The unusual property of four-membered ring peroxide heterocycles (1,2-dioxetanes) to produce electronically excited carbonyl fragments on thermal decomposition is discussed. The synthetic tools for the preparation of such labile substances and the analytic methods for assessing excitation yields are covered. Using the complete series of methylated derivatives of the parent 1,2-dioxetane, their thermal stability and excited state production is rationalized in terms of a modified diradical mechanism. With the help of bisdioxetanes, it is shown that generation of upper excited states is improbable, despite the fact that sufficient energy is available for this purpose. Attachment of dioxetanes to biomolecules via mild esterifications provides interesting opportunities for photobiological exploration.