SYNOPSIS. The isolation and small size of oceanic islands make them attractive models for studies of diversification; the sensitivity of their biota makes them important subjects for studies of extinction. I explore the origin of island biotas through dispersal and in situ diversification, and examine the fate of these biotas since human contact. Island biotas start out depauperate and disharmonic, facilitating the survival of relict taxa and stimulating adaptive radiations. The often highly restricted range and small population size of insular species, together with their limited diversity of defenses, make island biotas particularly vulnerable to extinction, largely through habitat loss or interactions with introduced species.