Cobscook Bay, a boreal, macrotidal estuary in the northeastern Gulf of Maine, is noted for its species richness. The Bay area is biogeographically complex due to present temperature regimes as well as historical, climatological, and physiographic changes since the last glaciation. Early investigators recognized the resulting zoological richness, and a significant portion of the collection and taxonomic description of North American marine fauna was centered in the northern Gulf of Maine. A compilation of the results of 19th- and 20th-century research leads to the conclusion that the Cobscook region contains the highest level of marine invertebrate biodiversity in eastern North America north of the tropics. In addition, Cobscook Bay is characterized by noteworthy biological attributes such as the intertidal occurrence of many normally subtidal species and the manifestation of giantism in several populations. These phenomena are summarized briefly as a prelude to a major ecosystem modeling effort.