Cold-seep communities associated with brine and hydrocarbon seepage on the upper Louisiana slope (ULS) of the Gulf of Mexico (GoM) were among the first discovered (Kennicutt et al., 1985), and they have become the most intensively studied and best understood of any cold-seep communities in the world. This recognition results from several factors: (1) There is a diverse funding base for this work, including the US Department of Interior Minerals Management Service, the National Oceanic and Atmospheric Administration (NOAA) National Undersea Research Program, the NOAA Ocean Exploration Program, and the US National Science Foundation, as well as a high level of interagency cooperation among these programs and with industry (see Kendall et al., this issue). (2) The relatively simple logistics of working at these intermediate-depth (320–800-m depth) sites, which are within 12 hours transit of a variety of US ports, has allowed near annual submersible visits to some of them for almost two decades of detailed, process-oriented studies. (3) Many of the animals from these seeps can be maintained alive in the laboratory at ambient pressure, and a variety of experiments with living animals has lead to a relatively detailed understanding of their physiology. (4) Oil, gas, and brine seepage is extremely widespread over a large portion of the GoM; over 90 sites from the base of the Florida Escarpment at 84°55´ in the east to 94°49´ in the west, and from 290-m to 3300-m depth have been confirmed by remotely operated vehicle (ROV) or manned submersible to harbor chemosynthetic macrofaunal communities (Figure 1). These factors have allowed a variety of research groups to develop and test the generality of hypotheses stemming from intensive study of a few sites, and to unravel the relative roles of depth, substrate characteristics, fluid expulsion dynamics, chemistry, and biogeography in structuring seep communities.