During the operational phase, building performance may decrease in various areas, so that the end users’ requirements are no longer met. Consequently, indicators are useful to assess and improve the performance of existing buildings. In this study, we carried out a literature review and organized a focus group with facility management experts to gather and analyze facility managers’ perceptions on operational indicators that could be used to assess the performance of buildings. The results revealed that the core indicators used to measure a building’s operational performance are related to safety and assets working properly, health and comfort, space functionality, and energy performance. The findings also revealed that these indicators can be obtained from three sources: a) facility managers/operators, who carry out corrective maintenance and perform technical inspections, b) regular users, who report complaints and fill-in satisfaction questionnaires, and c) sporadic users, who also fill-in satisfaction questionnaires. These indicators and their sources can contribute to a better analysis of building performance and the definition of measures to improve performance during the operational phase of a building.