Tremendous attention has been focused on the loss of and threats to, terrestrial biodiversity. Until very recently (Norse, 1993), much less attention was devoted to marine biodiversity despite the fact that marine systems are larger, older, have a huge impact on global climate, and support nearly twice as many phyla of animals as do terrestrial systems (see Table 2-2 in Norse, 1993). Our reduced attention to marine biodiversity reflects our relative ignorance of marine systems rather than their lack of importance to humans or to the ecosystem functions on which virtually all terrestrial life depends. Humans have spent thousands of years in intimate association with terrestrial biota, but only a few decades using SCUBA and submersibles to explore the world’s oceans. In this article we discuss how chemically mediated interactions affect marine biodiversity and consider the insights that can be gained from understanding the mechanisms involved in these interactions.