 We are grateful to the following individuals for very helpful feedback on earlier drafts of this entry: Lindley Darden, Melinda Fagan, Stuart Glennan, Phyllis Illari, Beate Krickel, Lucas Matthews, Irena Mikhalevich, Anya Plutynski, Gualtiero Piccinini, Mark Povich, and Katheryn Shrumm. We are also grateful to Pamela Speh for help with Figure 1.