•A comprehensive review of optimizing energy performance of existing buildings is presented.•Studies are discussed according to their decision-making model, measures and variables, software tools, etc.•A table of reviewed studies is presented that provides an overview of previous works at a glance.•Gaps for future works in this research area are outlined. Energy retrofit and renovation of existing buildings have an effective role in reducing carbon dioxide emissions and ultimately global warming. Therefore, there is an increasing interest in this field of research. The aim of this paper is to offer a complete, precise, up-to-date literature review on energy performance optimization of existing buildings. The literature is analyzed and discussed towards decision-making model, measures and variables, objectives and criteria, software tools, optimization methods, case studies geographical locations and case studies building types. Finally, trends and future research opportunities are presented. The results of the review show that in spite of numerous conducted studies, some optimization objectives such as aspects of comfort conditions, except thermal comfort, and some types of weather conditions have been neglected. Additionally, there is a need for creating a novel decision-making tool reducing computational time of optimization and increasing reliability on the optimal packages. This article provides an extensive overview for researchers to define their outlines in this area.