Models of Titan's thermal history indicate that an ocean consisting of an ammonia-water solution, as much as 200 km deep, is presently concealed beneath a crust of water ice. Conditions within this hypothetical ocean are discussed here and compared with conditions in terrestrial biomes. These conditions, while extreme by terrestrial standards, are such that life could indeed survive. This is in stark contrast with conditions on the surface where even the simplest prebiotic reactions have half-lives of the order 10 7 years, and free water would only infrequently be available to participate in chemical reactions. The plausible existence of life within Titan's subterranean ocean is discussed in terms of the effects that gaseous waste products of such indigenous organisms could have on the chemical and isotopic composition of the atmosphere. The possibility that all of Titan's current atmospheric CH 4 and N 2 is of biogenic origin is explored from a consideration of the potential productivity of oceanic microbes. While microbial life appears to be possible, it is unlikely that the Cassini/Huygens mission will be able to distinguish any unambiguous biomarkers. The existence and possible depth of the ocean may be constrained from Cassini flyby data, but detection of life in that ocean calls for a closer examination than the Huygens lander will afford us.