All life requires energy, which must be extracted from the environment. For all known life, free energy must be available at finite minimum levels in order to be usefully harnessed and must be delivered at finite minimum rates in order to support basic biochemical integrity and function. While seldom tested in the high energy light- and oxygen-based metabolisms of the surface biosphere, the magnitude of these requirements-the biological energy quantum (BEQ) and maintenance energy (ME) requirements, respectively-is considerable with respect to the potential metabolisms and energy sources that characterize the deep subsurface realm. As such, they constitute a fundamental constraint on the possible nature, distribution, and activity of microbial life in that environment. Because the energy released in a chemical transformation can be equated to the concentrations of substrates and products, both the BEQ and ME requirements define the minimum substrate concentration and minimum substrate production rate that must be sustained by a given environment for it to be capable of supporting life. The magnitudes of the BEQ and ME requirements are sensitive to a range of environmental parameters that may vary significantly in the subsurface. Temperature exerts a particularly strong control and is among the most important parameters to be considered in evaluating the energetic habitability of subsurface environments.