Energy transfer phenomena, in which excited fluorophores transfer energy to neighbouring chromophores, are well characterised in photochemistry and have found a wide range of applications in analytical biochemistry. The transfer of energy from a donor to an acceptor group is only significant over distances of a few nm, so it can be used as a spectroscopic ruler and as a means of detecting molecular interactions and conformational changes. Such methods usually retain the great sensitivity and sample handling flexibility of conventional fluorescence techniques. As a result many assays involving enzymes, antibodies and nucleotides utilise energy transfer measurement principles. This article outlines these principles for the main types of energy transfer, and summarises some of their most important areas of application.