Marine seeps are windows into different depth levels of the submerged geosphere. In global geologic settings the sources of seep fluids range from 10s of meters (groundwater aquifers) to 10s of km (subducted oceanic plates) below the seafloor. At the sedimented active and passive margins organic-rich and evaporite-containing strata determine the final fluid composition, emitting characteristically gashydrate-derived methane and brine-associated non-methane hydrocarbons, respectively. Seeps transport dissolved and gaseous phase compounds to the ocean and sustain oasis-type ecosystems at the seafloor by providing bioactive reductants sulfide, methane and hydrogen. The interaction between fluid composition, fluxes and biota results in a diagnostic hydrocarbon-metazoan-microbe-carbonate association of which currently >80 are active globally. The single most important reaction is anoxic oxidation of methane by Archaea (AOM) with secondary reactions involving thiotrophy and carbonate mineral precipitation. Function, structure and composition of AOM-consortia and metazoan assemblages in concert with the characterization of biomarkers are overwhelming topics of seep research with increasingly frequent identification of ancient seep sites (>300 million years ago). The library of biomarkers as well as fossilized microbial bodies grows steadily aided by the fortuitous situation that both are preserved in carbonate precipitates. The Gulf of Mexico, the Black Sea and the eastern Mediterranean Sea are sites of classical and ongoing seep studies. Large-scale new studies of seeps by the national consortia have been initiated in the Indian Ocean, the South China Sea and the Eastern Sea off Korea where gas hydrate layers are being drilled in the search for new energy.