Approximately 550 million tonnes of the monoterpene, isoprene, are emitted to the atmosphere annually, principally from terrestrial plants. In contrast to methane, which is emitted in similar quantities, little is known about the biodegradation of isoprene. However, 30 years ago, bacteria capable of living on isoprene as a sole source of carbon and energy were described, although they were not investigated in detail. Recently there has been renewed interest in the potential of bacteria living in soils, marine sediments, and on the leaves of plants to degrade isoprene. Isolates capable of isoprene metabolism use a multicomponent soluble monooxygenase, which contains a diiron center at the active site, to oxidize isoprene to the epoxide, and all isolates described to date depend on glutathione for subsequent metabolic steps. The diversity of isoprene degraders has been investigated in terrestrial and marine environments using DNA-stable isotope probing (DNA-SIP), together with the use of gene probes targeting the monooxygenase active-site subunit. Gaps in our knowledge and future research directions are described.