Many archaea and bacteria obtain metabolic energy by catalyzing the oxidation or reduction of sulfur. In marine hydrothermal systems, chemolithoautotrophs that oxidize hydrogen sulfide (H₂S) or elemental sulfur (S⁰) account for much of the primary biomass synthesis. Under reducing conditions in these systems, both S⁰ and sulfate can serve as terminal electron acceptors. The energetics of chemolithoautotrophy in marine hydrothermal systems are discussed, focusing principally on sulfur-redox, but also touching on methanogenesis and organic synthesis. Examples are given from deep- and shallow-sea hydrothermal environments, the early Earth, Mars, and Europa. In addition, we present a detailed analysis of the Gibbs free energies (δG _r ) of 25 sulfur-redox reactions in a model shallow-marine hydrothermal ecosystem—the seeps, wells, and vents of Vulcano Island (Italy). A number of these reactions represent known metabolisms, but other reactions with no known microbial catalyst are also included to investigate their potential as pos sible energy sources. The reactions considered couple SO²⁻ ₄ S⁰, and H₂S with a variety of terminal electron acceptors (O₂, NO⁻ ₃, Fe(III), CO₂) and electron donors (H₂, CH₄, formic acid, acetic acid, propanoic acid, NH⁺ ₄, Fe²⁺). At all seven study sites on Vulcano, which vary considerably in temperature, pH, and chemical composition, sulfate- and S⁰-reduction reactions are energy-yielding where H₂, CH₄, or carboxylic acids serve as the electron donors, but energy-consuming with NH⁺ ₄ or Fe²⁺ as the reductant. Elemental sulfur- and sulfide-oxidation reactions are energy-yielding at all sites when O₂, NO⁻ ₃, or Fe(III) are the terminal electron acceptors, but energy-consuming with CO₂ as the oxidant.