In recent years the development stages of Copepoda have been the subject of study by various workers. Their attention, however, has been mainly directed to fresh-water species, presumably because the material is more readily available, and it seems desirable that our knowledge of the development of marine Copepoda should be extended. The material utilised for the present work was collected on the periodic and comprehensive fishery cruises carried out by the F.B.S. Explorer, and we are indebted to the Fishery Board for Scotland for permission to undertake this detailed study. Since the appearance of Grobben's standard work on Cetochilus septentrionalis Goodsir (Calanus finmarchicus Gunn.), the only important publication dealing with the life-histories of marine Copepoda, so far as we have been able to ascertain, is that of Oberg, who, in Die Metamorphose der Plankton-Copepoden der Kieler Buclit, describes the development stages of seven common marine forms. Oberg's choice of species, which are all more or less neritic, was again obviously dictated to him by the material at his disposal, and it is hoped that our collections, which cover not only the northern part of the North Sea, but also the Faroe–Shetland Channel, and part of the Atlantic itself, will furnish material for the establishment of the life-histories of some of the more oceanic forms.