One of three major ecological groups into which marine organisms are divided, the other two being the nekton and the plankton. The benthos are organisms and communities found on or near the seabed. This includes those animals (zoobenthos) and plants (phytobenthos) living on (epifauna) or in (endofauna) marine substrata as well as those that swim in close proximity to the bottom without ever really leaving it. In terms of size, this is generally divided into three categories: meiobenthos, the organisms that pass through a 0. 5 mm sieve; macrobenthos, those that are caught by grabs or dredges but retained on the 0. 5 m sieve; and epibenthos, those organisms than live on rather than in the seabed. Those in the latter category are usually larger.

Benthic life is subject to vertical zonation depending chiefly on light, moisture and pressure. This has led to the division of benthonic animals into two systems and seven zones. Proceeding from shallow to deep water, the first system is the phytal or littoral system, composed of the supralittoral, mediolittoral, infralittoral and circalittoral zones. The second system, the aphytal or deep system, is composed of the bathyal, abyssal and hadal zones.