The update published on August 4, 2017 contains new Section 3, written by T. Parr. Many thanks to B. Baertschi, P. Bou-Habib, M. Clayton, P. Destrée, P. Dietsch, B. Engelen, R. Gannett, A. Kahan, O. Klein, L. Langlois, C. Larrère, C. Mills, D. Naurin, T. Pogge, H. Pourtois, P. Pross, A. Roberts, M. Schwartzman, A. Walton, and A. Williams for very valuable suggestions and information. Earlier versions of this entry were presented at the universities of Aarhus (2016), Montréal (2003), Bristol (2004), Louvain (2004) and St Louis (Brussels) (2004). We wish to thank these audiences. The usual disclaimers apply.