Although it is becoming clear that microorganisms are abundant in marine deep sediments [1–8], it is unclear what percentage of cells are active, how fast they are growing or what controls their diversity and population size . Addressing these issues is a formidable task due to the relative inaccessibility of these environments, the difficulty of cultivating representative microorganisms and the long time scales associated with some of their lifestyles [2, 10–12]. However, quantitative limits on life in the subsurface can be determined by using the physiochemical data that describe their habitats. In particular, the chemical composition can be used to constrain likely metabolic strategies and rates in a given setting. This is accomplished by calculating values of Gibbs energy available from reactions containing different combinations of the electron donors and acceptors that are found in these environments. Not only can Gibbs energies of reaction reveal which catabolic strategies are thermodynamically possible, but they can also help determine which geochemical variables (e.g. temperature, pressure, pH, salinity, composition) are controlling microbial activity. When reduced to an environmentally-appropriate common factor, the energetic potential of all biogeochemical environments can be directly compared to assess how energy limitations affect the amount and type of biomass in them. In the present chapter, geochemical data obtained from sediment cores taken from the Peru Margin, South Pacific Gyre and Juan de Fuca Ridge are used to assess the Gibbs energies of plausible catabolic strategies including, but not limited to, the oxidation of organic matter, methane and hydrogen by a variety of electron acceptors. In conjunction with cell-count data, the results of these calculations illustrate the importance of normalizing energy availability to the limiting substrate and how geochemical data can be used to better understand the distribution of life deep in marine sediments.