The methanogenic biodegradation of crude oil is an important process occurring in many subsurface hydrocarbon-associated environments, but little is known about this metabolism in such environments. In this thesis work, the methanogenic biodegradation of crude oil and polycyclic aromatic hydrocarbons (PAH) was investigated. Methanogenic cultures able to metabolize light and heavy crude oil components were enriched from oilfield produced waters. Metabolites (e.g., alkylsuccinates) and genes (e.g. assA and bssA) associated with a fumarate addition mechanism were detected in the light oil-amended culture. A Smithella sp. dominated the community, suggesting this organism was involved in the degradation of the hydrocarbon components. In experiments conducted in sandstone-packed column systems simulating marginal oil fields, the light oil-amended culture was shown to bioconvert alkanes and aromatic hydrocarbons to CH4. Other oil-associated microbial inocula also enhanced CH4 production from oil in the column systems. Shifts in the microbial communities were observed after the inocula were incubated in the columns. Methanogenic hydrocarbon metabolism was also investigated using new enrichment cultures that biodegraded 2-ringed PAHs under methanogenic conditions. Metabolite and marker gene analyses were conducted on these cultures to investigate the mechanism(s) involved in PAH metabolism. The PAH-utilizing enrichments were dominated by methanogens closely affiliating with Methanosaeta and Methanoculleus, and bacterial members most closely related to the Clostridiaceae family. Further qPCR analysis with a 2-methylnaphthalene-amended culture suggested that Clostridium was the main hydrocarbon degrader in the enrichment. The results of these studies have added new knowledge to the field of methanogenic hydrocarbon biodegradation that may find application in bioremediation or microbial enhanced energy recovery.