Optimization theory and methods have been applied in many fields to handle various practical problems. In light of advances in computing systems, optimization techniques have become increasingly important and popular in different engineering applications. The aim of this special issue is to present some recent developments in the area of optimization theory, methods, and applications in engineering. This special issue involves 44 original papers, selected by the editors and related to the various researches themes on optimization theory, methods, and applications in engineering. According to the characteristics of the accepted papers, the special issue is organized in the following six parts and each part is composed of several important papers to the part’s scope.