Systematic collections have revealed that the Madang Lagoon is a highly heterogeneous reef system which shows an unprecedented diversity of marine invertebrates (e.g., about 180 species of gammaridean Amphipoda). The characteristics of the Madang Lagoon, including its geological origin, physical geography, and physical and biological oceanography, are described and discussed. Traditional names are used for the reefs and islands of the lagoon. An appendix includes a list of all current collecting localities for the Madang Lagoon amphipod project. Coordinates for these collecting localities have been determined using a global positioning device.