Professor Wilson Baker, F.R.S. Organic compounds are classified as aliphatic, carbocyclic, or heterocyclic, though in the very many cases where two or more such characteristic groupings are present, the classification chosen will depend on the relative chemical importance of these groupings to the particular investigation in hand, and perhaps even to the outlook of the investigator. Traditionally, however, ring compounds with attached aliphatic groups are referred to as cyclic, and any hetero­ cyclic grouping serves to categorise a molecule as heterocyclic. In these reviews it is the intention to deal, so far as possible, with carbocyclic compounds only, as borne out by this Volume I of the series with articles on the benzidine rearrangement, the bicyclo­ [3,3,1]nonanes, Feist's acid, and the annulenes. The difficulty in keeping rigidly to carbocyclic substances is, however, apparent in the chapter on 'The Biosynthesis of Carbocyclic Compounds', where many heterocyclic compounds are encountered, as is inevitable in any reasonably comprehensive account of biosynthesis.