 I have benefited from discussing both the organization and content of this article with many people including audiences and participants at the 2009 Pittsburgh Workshop on Underdetermination and the 2009 Southern California Philosophers of Science retreat, as well as the participants in graduate seminars both at UC Irvine and Pittsburgh. Special thanks are owed to John Norton, P. D. Magnus, John Manchak, Bennett Holman, Penelope Maddy, Jeff Barrett, David Malament, John Earman, and James Woodward.