Abstract The continental shelves are discussed as a world-wide phenomenon. Topographically they are relatively flat, though off most coasts they have numerous terraces and small hills, and off glaciated coasts they have relatively deep troughs and basins. The sediments of the shelves show a surprising lack of an orderly decrease in grain size with increased distance from the coast. In general, the coarsest sediment is found along the outer margins of the shelves or directly adjacent to the coasts. Rocky bottom is reported so commonly on the outer shelves as to indicate that these platforms do not represent great sedimentary accumulations built out beyond wave-cut terraces. Also, it appears from the nature of the sediment that shelves do not represent areas where much deposition is occurring at the present time. In view of the inaccuracy of the sources of information, discussion of the exact physical, chemical, and organic nature of the sediments is left to the papers that deal with specific areas from which samples are available. Also, the sources and transportation of the recent shelf sediments will be dealt with in subsequent papers.