We are unique. Although all species affect their environments, Homo sapiens affects more of our planet in more ways to a greater degree than any other species, past or present. But we are also unique in another way that could be our salvation: we can recognize changes harmful to our interests and consciously act to ameliorate or even reverse them. The growing movement to conserve marine biological diversity is humankind’s first comprehensive response to the damage we inflict on the world’s estuaries, coastal waters, and oceans. Because the marine biodiversity movement is a work-in-progress, to understand where it is going requires an understanding whence it has come.