Photochemical reactions and processes involving non-absorbing substrates do not occur without the intervention of a light-absorbing substance known as a photosensitiser. The sensitiser absorbs the light, raising its energy to the excited singlet state, which then undergoes intersystem crossing to produce a relatively long-lived, excited triplet state. The sensitiser can undergo chemical reactions, and is then unavailable for return to the original ground state. Alternatively it can behave as a photocatalyst, reacting with other molecules either by photoinduced electron transfer (ET) or triplet–triplet energy transfer (excitation transfer), or H-atom transfer (HAT), before returning to its singlet ground state for recycling. The role of the sensitiser, in most cases of large-scale industrial significance, is to initiate the main reaction.