The remainder of this report, with the exception of Conclusions and Recommendations, consists of edited proceedings of the workshop conducted by the committee on May 16, 1996, in Washington, D.C. The report is divided into sections, the central features of which are prepared addresses by invited experts selected by the committee at a planning meeting in February. Each invited talk is preceded by a brief introduction written by a committee member and followed by one or more brief commentaries from predesignated "reactors" and by excerpts from the lively discussion that ensued among speakers, reactors, committee members, and a small number of other invited participants. Brief biographies of the committee members and speakers, and a list of other participants, are included in the appendix material. The invited speakers and reactors were all asked to check the text for accuracy, and the opinions expressed are solely those to whom they are attributed. The final section is the work of the committee alone and brings together the committee's conclusions and recommendations.