This volume contains the edited texts of the lectures presented at the Workshop on Nonlinear Optimization held in Erice, Sicily, at the `G. Stampacchia' School of Mathematics of the `E. Majorana' Centre for Scientific Culture, June 23-July 2, 1998. In the tradition of these meetings, the main purpose was to review and discuss recent advances and promising research trends concerning theory, algorithms and innovative applications in the field of nonlinear optimization, and of related topics such as convex optimization, nonsmooth optimization, variational inequalities and complementarity problems.