This review is concerned with the group of enzymes known as 2-oxoglutarate-dependent dioxygenases. Over the past 25 years, a number of enzymes have been identified with the cofactor and co-substrate requirements that typify this group. More recently, the genes encoding some of these enzymes have been cloned and sequenced and it is clear that the classification of this enzyme family is more complex than originally thought. The last comprehensive review of this area was published in 1974 and progress since then more than merits a re-evaluation of the relationships between the enzymes regarded to be members of the biochemically-defined, 2-oxoglutarate-dependent dioxygenase family and other enzymes which are not 2-oxoglutarate-dependent, but which are related by virtue of sequence homology or mechanism of action or both. Thus a larger group of 2-oxoacid-dependent oxygenases which includes the 2-oxoglutarate-dependent dioxygenases is emerging.