I am grateful for suggestions, comments, and contributions from the following friends and colleagues: A. Andrews, L. Beese, C.L. Brooks III, R.L. Campbell, G.M. Clore, F. Colonna, W. Delano, M. Desrochers, E. Dodson, T. Garrett, A.M. Gronenborn, J. Habazettl, T. Holak, R. Hubbard, T. Hynes, J. Jiansheng, M. Karplus, M. Kearney, E. Kim, A. Krukowski, J. Kuriyan, J. Kuszewski, M. McCallum, M. Nilges, S. O'Donoghue, D. Perahia, G.A. Petsko, J.W. Pflugrath, M. Reifler, S. Reifler, L. Rice, D. Ringe, T. Schneider, P. Schultze, T. Simonson, D.L. Smith, H. Treutlein, W.I. Weis, and H.W. Wyckoff. I also thank the many users of X-PLOR for their patience with earlier versions and their valuable suggestions. Development of X-PLOR was supported in part by the Howard Hughes Medical Institute, the National Science Foundation (DIR-9021975), and Molecular Simulations Incorporated.