This volume comprises the proceedings of a NATO Advanced Study Institute held in Geilo, Norway, between 8-19 April 1985. Although the principal support for the meeting was provided by the NATO Committee for Scientific Affairs, a number of additional sponsors also contributed, allowing the assembly of an unusually large number of internationally rec- ognized speakers. Additional funds were received from: EXXON Research and Engineering Co. IBM (Europe) Institutt for energiteknikk (NorwaY) Institut Lauge-Langevin (France) The Norwegian Research Council for Science and Humanities NORDITA...