Chapter 5 commences with a definition of adaptive reuse and conversion and gives a broad review on existing adaptive reuse theory. Adaptive reuse has been studied and described internationally, from different perspectives and disciplines. This chapter combines these different studies, developing a theoretical framework for adaptive reuse. One of the main drivers for adaptive reuse, next to common adaptation drivers, is building or functional obsolescence. Functional obsolescence is caused by societal, economic and technological changes, based on which the building is no longer suited to accommodate its original function. Over-production and under-occupancy of offices are a specific cause for obsolescence that is discussed in this chapter.