This work utilized a multidisciplinary approach to explore the microbial biogeochemistry of methane cycling in the marine environment with the aim of identifying which processes were occurring and which microorganisms were involved, focusing on two methane-rich systems: oil-laden, gas hydrate-bearing and/or brine-charged cold seeps in the Gulf of Mexico and the Hydrate Ridge deep biosphere. The research presented here expands our knowledge of microbially-mediated anaerobic oxidation of methane (AOM) and associated process such as sulfate reduction (SR) and methanogenesis in surficial and deep methane-rich environments. This work confirmed that the ANME-1 and -2 clades of methanotrophic archaea are responsible for AOM in surficial sediments in the Gulf of Mexico and was the first to demonstrate that the ANMEs are also involved in the production of methane, although at a fraction of the rate of AOM. Our work was the first to directly document microbial activity within gas hydrate material collected from the Gulf of Mexico, which suggests that microbial activity may impact the biogeochemical cycling of methane within the unique gas hydrate niche. Active populations of both Bacteria and Archaea were observed in a methane-rich deep biosphere environment, in contrast to previous studies which suggest that one or the other domain is dominant. In methane-rich areas of the deep subsurface, ANME were detected for the first time in an area where sulfate levels were elevated, although AOM may be limited in the deep biosphere by the availability of sulfate coupled with slow growth rates. Unlike at other cold seeps which have been studied to date, SR in Gulf of Mexico surficial sediments is often uncoupled from AOM and is also fueled by other endogenous hydrocarbons and petroleum derivatives; the identity of the microorganisms which mediate sulfate-dependent hydrocarbon oxidation in situ is unclear but likely includes members of the seep-endemic Deltaproteobacteria sulfate reducing bacterial (SRB) clades. Modeling approaches to determine the potential intermediate compound exchanged within consortia of ANME and SRB to sustain AOM and SR revealed that hydrogen, acetate and formate cannot sustain rates of activity that match measured values due to diffusion-limited removal of the intermediate compound. More information is available online at http://alice508.myweb.uga.edu/ or by emailing <alice508@gmail.com>