Utilization of marine resources is the oldest impact on the ocean environment by humans: with widespread development of fisheries, it has become the major ecological impact. Humans have fished since before recorded history. Over the last 50 years, the global marine catch has increased approximately fourfold, to >80 million tons, but the rate of increase has slowed (FAO, 1993). Historically, fisheries exploited near-shore and coastal resources, but the expansion of far seas fisheries in the second half of the 20th century led to exploitation in all the world’s oceans (Garcia and Newton, 1994). Thus it is little wonder that a National Research Council workshop identified “fisheries operations” as the most important anthropogenic effect on marine biodiversity among five major critical environmental issues (NRC, 1995) and suggested taking a regional approach to examine human impacts on marine biodiversity, consistent with the broad geographic range of fisheries. A general definition of biodiversity is “the collection of genomes, species, and ecosystems occurring in a geographically defined region” (NRC, 1995); fisheries impact baseline diversity at each of these levels. At the genetic level, fisheries change population characteristics (e.g., age distribution, reproduction, stock structure), resulting in alterations to the genome. At the species level, fisheries affect species composition and interactions. Finally, through effects of by-catch, habitat alteration, and altered energy flow, fisheries impact the diversity of marine habitats and the function of ecosystems. In this paper, I describe how marine fisheries affect biodiversity and discuss fisheries management in light of the importance of biodiversity to the sustainability of marine fisheries.