This chapter provides a general introduction to the ecosystem of the deep-sea floor, beginning with a description of the physical environment of the deep sea. A section on how information is obtained about the deep-sea-floor ecosystem follows, because knowledge of this ecosystem is greatly influenced by the effectiveness of the available technology. Introductions to the fauna of the deep sea where the substratum is sediment (soft bottoms) and where it is not (hard bottoms) follow. The chapter concludes with a section on the pace of life in the deep sea.