Sustainability is a term frequently used across different working fields. However the word seems to have lost some of its meaning as it has been over-used.The building sector accounts for a large amount of the energy-use in Sweden and there are a lot of improvements to be made in this area. Lately an interest in environmental certification of buildings has developed. There are some advantages with these environmental certification systems as well as some disadvantages. The various actors in the building process experience these advantages and disadvantages differently. The objective of this thesis is to examine why investors are willing to use environmental certification of buildings, when it is widely known that there are additional costs to classify for these certificates. The question why some investors do not want to certificate their buildings is also investigated. Further the function of environmental rating systems for an architectural firm is also studied.The starting-point of this master thesis is a literature review related to these topics. The empirical data for the study was collected through a questionnaire and in-depth interviews. Three interviews with investors as well as interviews with an architectural firm were conducted. The result of these studies show that environmental certifications are not as widely used as thought among the investors. A few of the asked investors had taken a decision to use environmental certifications. One of these investors was asked in an interview what advantages they saw with this kind of certification systems and answered that it is strengthening their brand as well as it is sustainable in the long term. Most of the responding investors in the questionnaire were not interested in this kind of certification because of the additional costs connected to the certification. They argued that they could build as good, or even better, buildings as the certificated ones without having to pay for the certification fees and expensive building materials required. The study also showed that the advantages that were connected to environmental certification were not always easy to measure. For example social responsibility and good will. The greatest reason why not to certificate was the rising costs.