Subaqueous slope and base-of-slope depositional systems are a major component of most marine and many lacustrine basin fills, and constitute primary targets for hydrocarbon exploration and development. Seven basic facies building blocks comprise slope systems: (1) turbidite channel fills, (2) turbidite lobes, (3) sheet turbidites, (4) slide, slump, and debris-flow sheets, lobes, and tongues, (5) finegrained turbidite fills and sheets, (6) contourite drifts, and (7) hemipelagic drapes and fills. The grain size of supplied sediment is a primary control on channel and lobe morphologies and on the scale and importance of slump and debris-flow deposits. Two general families of siliciclastic slope systems occur. Constructional (allochthonous) systems, including fans, aprons, and basin-floor channels, are built of sediment supplied from superjacent delta, shore-zone, shelf, or glacial systems. The facies architecture of allochthonous systems is determined jointly by the sediment texture and pattern of supply to the shelf margin. Point sources of supply create fans; line sources create strike-elongate prisms of slope sediment called slope aprons. Shelf-margin deltas provide a particularly common intermediate source geometry, forming offlapping delta-fed aprons. Autochthonous systems, including retrogressive aprons, canyon fills, and megaslump complexes, record slope reworking and resedimentation.