The book "Nonlinear Optimization and Related Topics" offers a comprehensive collection of lectures that were held during the workshop on nonlinear optimization in Erice, Sicily. This workshop took place in June and July 1998 at the G. Stampacchia School of Mathematics of the E. Majorana Center for Scientific Culture. The aim of the event was to review and discuss current advances and promising research trends in the theory, algorithms, and innovative applications of nonlinear optimization, as well as related topics such as convex optimization, nonsmooth optimization, variational inequalities, and complementarity problems. With the participation of 83 professionals from 21 countries, a broad and deep understanding of the current research trends in this field was achieved. The contributions of the participants and the support of the Ettore Majorana Center significantly contributed to the success of the workshop.