This volume contains the proceedings of the NATO Advanced Study Institute held at Centro di Cultura Scientifica "A. Volta," Villa Olmo, Como, 25 June -6 .J uly 1990. R. Artuso, University of Milano, was the scientific secretary of the Institute, the director was P. Cvitanovic, Niels Bohr Institute, while G. Casati, Universit.y of Milano, acted as the host and the co-director. Other members of the scientific organiz ing committee were R.E. Ecke, Los Alamos National Laboratory, M.J. Feigenbaum, Rockefeller University, and 1. Procaccia, Weizmann Institute. The attendence at the school consisted of 20 lecturers and 89 students. The term "student" covers here a broad range from a graduate student to a well.established professional, and indeed the Best Student prize was won by Eddie G.D. Cohen, a student well advanced. The organizers of the school would like to thank H.H. Rllgh and R. Mainieri for running the very lively "student" seminar series, to our hosts at Villa Olmo for making our stay so pleasant, to the lecturers and seminar speakers for their valiant efforts to enlighten us, to Dipartimento di F'isica, Universitcl di Milano for additional funding, and to R. Artuso for making this school a success. The Feigenbaum lectures were written up by Z. Kovacs, while A. Oliveira and S."