Many buildings which were built in the 20th century, and due to their exceptional architectural value included in the lists of built heritage, are sometimes standing vacant for different reasons. This paper investigates the problems that need to be resolved to enable a sustainable reuse of various types of modern heritage buildings. The investigation is undertaken through case studies of some modern heritage buildings in Scotland and Italy in order to identify common problems and regional differences in enabling the reuse of those buildings. In addition, some examples of the reuse of modern built heritage are presented to highlight what has contributed to the reuse, and whether and how that meets the current environmental requirements, and the local social and economic needs. The research indicates how public and private organisations have contributed to the successful reuse of modern built heritage and what problems they encounter in the efforts to provide new uses for the remaining vacant buildings. The investigation examines how economic, social, environmental, functional, structural and design aspects impact on defining new uses for modern heritage buildings. The analysis of the above requirements through the selected case studies leads to the recommendations on the key issues, strategies and tactics that should be considered to enable an appropriate and timely reuse of the 20th century built heritage.