The potential detrimental effects on human health and pervasive distribution of polycyclic aromatic hydrocarbons (PAHs) have drawn increased attention to their biodegradation mechanisms. A variety of sources, including atmospheric fallout, combustion of fossil fuels, industrial runoff, petroleum spills, automobile exhaust, and urban discharge, can release PAHs into aquatic environments. Due to the poor water solubility of PAHs, sediment acts as the ultimate sink for PAH deposition which makes the coastal and marine ecosystems most vulnerable to PAH accumulation. Microbial communities majorly participate in controlling the fate of PAHs, thereby influencing the biogeochemical cycling of carbon in water and sediments. Microbes of coastal and marine ecosystems are deemed to have intrinsic property of PAH degradation owing to their rich diversity and unique adaptations. This chapter summarizes the recent developments in microbial degradation of PAHs in both marine and coastal ecosystems.