Delayed chlorophyll fluorescence (DF) is 8 ligfit ímission emitted by plants, algae and photosynthetic bacteria immediately after their irradiatíon, originating ffom chlorophyll-pigment complexes and permitted by the entire entity of the photosynthetic apparatus. Although DF presents insignificant loss of energy potentially ušed in photosynthesis process, it is a very useful and non-invasive tool to study the "light phase" of photosynthesis. The review describes the DF phenomenon, offers latest theories that explain it, presents current knowledge of its kinetic behaviour, and puts speciál emphasis on its temperature dependence and eventual purpose. Finally, possible developments in future DF research are predicted.