Current research is reviewed on the photoreactions that occur when sunlight interacts with soil and aquatic organic matter. The primary focus is on photoprocesses involving humic substances. Investigations of the direct photoreactions of humic substances are discussed, with emphasis on the fading of the uv-visible and fluorescence spectra. The balance of the background paper deals with sunlight-induced reactions between humic substances and trace organic or metallic substrates. Recent studies have shown that these indirect photoreactions are mediated by electronically excited states or reactive intermediates derived therefrom that react with substrates by electronic energy transfer, by electron transfer, and/or by various free radical processes. Concepts and methods that are used to elucidate the nature and concentrations of these transient reactants are discussed, including approaches based on steady-state irradiations and techniques that employ laser flash photolysis.