One of the key challenges in modern Facility Management (FM) is to digitally reflect the current state of the built environment, referred as-is versus as-designed and as-built representation. While the use of Building Information Modeling (BI M) can address the issue of digitalrepresentation, the generation and maintenance of BIM data requires a considerable amount of manual work and domain expertise. Another keychallenge is being able to monitor the current state of the built environment, which is used to provide feedback and enhance decision making.The need for an integrated solution for all data associated with the operational life cycle of a building is becoming more pronounced as practicesfrom lndustry 4.0 are currently being evaluated and adopted for FM use. This research presents an approach for digital representation of indoor environments in their current state within the life cycle of a given building. Such an approach requires the fusion of various sources of digital data. The key to solving such a complex issue of digital data integration, processing and representation is with the use of a DigitalTwin (DT). A DT is a digital duplicate of the physical environment, states, and processes. A DT fuses as-designed and as-built digitalrepresentations of built environment with as-is data, typically in the form of floorplans, point clouds and BIMs, with additional informationlayers pertaining to the current and predicted states of an indoor environment or complete building (e.g., sensor data). The design, implementation and initial testing of prototypical DT software services for indoor environments is presented and described. These DT software services are implemented within a service-oriented paradigm, and their feasibility is presented through functioning and tested key software components within prototypical Service-Oriented System (SOS) implementations. The main outcome of this research shows that keydata related to the built environment can be semantically enriched and combined to enable digital representation s of indoorenvironments, based on the concept of a DT. Furthermore, the outcomes of this research show that digital data, related to FM andArchitecture, Construction, Engineering, Owner and Occupant (AECOO) activity, can be combined, analyzed and visualized in real-time using aservice-oriented approach. This has great potential to benefit decision making related to Operation and Maintenance (O&M) procedures within the scope of the post-construction life cycle stages of typical office buildings.