The Earth's wealth of genetic, species, community, and landscape-level diversity is disappearing at an unprecendented rate because of humanity's ever-accelerating alteration of habitat. The loss of biodiversity in the aquatic environment is now recognized as a serious concern by aquatic biologists. Development and use of our natural resources have been practiced in an unsustainable manner. Political institutions have been challenged to reconcile competing private and public goals, as well as a diverse set of expectations of our natural resources, including aquatic systems. This paper defines biodiversity, identifies many values of diversity in the biosphere, and discusses both proximate and ultimate causes of its decline.