- Post-occupancy evaluation (POE) is a platform for the systematic study of buildings once occupied, so that lessons may be learned that will improve their current conditions and guide the design of future buildings. Various aspects of the occupied buildings’ functioning and performance can be assessed in a POE, both chemo-physical (indoor environment quality (IEQ), indoor air quality (IAQ) and thermal performance) as well as more subjective and interactional (space use, user satisfaction, etc.). POE draws on an extensive quantitative and qualitative toolkit: measurements and monitoring, on the one hand, and methods such as walk-throughs, observations and user satisfaction questionnaires on the other. POE may seem a necessary, indeed, axiomatic phase of the design and construction process, and exactly the kind of integrated assessment essential for the design of more sustainable buildings. Yet POE researchers have often been regarded with suspicion and even hostility, since their work may cause friction between different stakeholders. This chapter reviews material published in recent years in an attempt to trace the emergence of POE, describe its conceptual and methodological backdrop, its interaction with other issues related to sustainable design, and its increasing ‘canonization’ as a method. We argue that POE offers the potential to integrate a range of fragmented aspects of the construction process and of the relations of buildings to their environment and users. We propose that the acceptance of POE as a mandatory step in the design and commissioning of buildings, whose results are habitually fed backward and forward to other stages of the design and construction processes, is an important and probably inevitable step toward making buildings more sustainable.