This proceedings volume contains 38 papers presented at the 4th Working Conference on "Rel iability. and Optimization of Structural Systems," held at the Technical University of Munich, Germany, September 11- 13, 1991. The Working Conference was organised by the IFIP (Interna tional Federation for Information Processing) Working Group 7.5 of Technical Committee 7 and was the fourth in a series, following similar conferences held at the University of Aalborg, Den mark, May 1987, at the Imperial College, London, UK, September 1988 and at the University of California, Berkeley, California, USA,...