This is a report on a workshop held at SLAC on September 23 and 24, 1999 to aid the detailing of the LCLS FEL Physics R&D for the FY2000 to FY2002 period. This report consists of an Executive Summary and Summaries by the speakers of each of the two working groups. The workshop program and copies of most of the viewgraphs that were shown are attached. Also included are summary descriptions of the five major FEL simulation codes. The summaries are written by the code authors.