The usage phase of a building accounts for a significant part of the Swedish real estate sector's environmental impact, which makes existing properties an important area to work with to establish the climate footprint of the real estate sector. Various policy instruments have been developed in line with this, of which the environmental certification system has become widespread, but mainly for newly constructed buildings. By conducting a literature study and an interview study directed at representatives of a selection of Swedish real estate companies, the current study aims to answer the following two questions:  -       What are the driving forces and motives for real estate companies to invest in environmental certification of existing buildings? -       What added values and challenges do expert representatives from real estate companies see regarding environmental certification of buildings during the usage phase? The respondents in the interview study are all active at Swedish real estate companies and represent different professional roles and expertise in the areas of environment, investment or property management.  The interview study confirms several of the motives, driving forces, added values and challenges that have been pointed out in the literature, but also shows that differences exist between respondents and within different areas of expertise. Overall, there was a positive picture of implementing environmental certification systems for existing buildings. The respondents’ common motive for applying environmental certifications for existing buildings was the demand from tenants. The certification work in existing buildings generates an added value of control and quality assurance as well as a communicative value to the tenant, according to the interviews. The main challenges with the certification work was the increased workload and need for access to internal resources in terms of manpower and time, the extent of which vary with the varying state of the buildings. There was a number of identified added values, for instance how the certification cost for an existing property was perceived as less expensive compared to that of certifying a new building. The author interprets that the main driving force behind the implementation was the growing interest in environmentally certified buildings among tenants, and that this is a powerful impact that could be further utilized. In conclusion, there are several underlying motives and driving forces for real estate companies to apply environmental certification systems for existing buildings, but the perceived utility depends on who is consulted and their professional role. The literature study and the empirical evidence show that ecological and economic benefits exist, but also that environmental certifications of existing buildings contribute to practical and administrative benefits. While the certification systems were associated with a number of practical and economic challenges, their extent was lower than expected. From an academic perspective, the study is important as new information about the use of environmental certification systems has been discovered, which may affect future development of the systems. Outside the academic perspective, the study can be used as a guide for, for example, real estate companies and consultants prior to environmental certification of existing buildings, which is important as it can mean savings of internal resources.