Various highly significant linear correlations between the duration of the intermoult period and the growth factors at moulting on the one hand and body length, weight and successive moult number on the other are defined in decapod crustaceans. Log body weight or L3 is linearly related to log age. The presence of such correlations describing the growth and moulting in other groups of Crustacea is discussed. The application of the various regression equations to the study of growth and life histories of commercial species is described.