The aim of this thesis is to understand the ongoing professionalization of sustainability by problematizing the role and work of sustainability professionals in the Swedish construction industry. Building on a practice lens, the research is based on a qualitative research design, including interviews with sustainability professionals and an in-depth case study of a large infrastructure project. The findings show that although sustainability roles have existed in the industry for about 30 years, a stable and clearly defined foundation for their work is still lacking. This is demonstrated by the way their work constantly changes direction depending on the sustainability issue “in fashion”, and by how they adapt their roles and practices depending on the situation and with whom they interact. This contributes to the fragmented and continually evolving nature of sustainability roles. Sustainability professionals also take on significant responsibility for trying to create clarity around “who should do what” in sustainability work. This is demonstrated through their boundary work, aimed at influencing the boundaries between different professional groups’ practices. In doing so, however, they are continuously reshaping their own role, which risks reinforcing the lack of clarity regarding what sustainability professionals actually do, and should do, within the company or in construction projects. This thesis contributes by developing an empirical and theoretical understanding that builds on practice, that is, what professionals do, as a way to understand the ongoing professionalization of sustainability by problematizing the role and work of sustainability professionals in the Swedish construction industry. To theory, this thesis contributes by providing rich details about what it means to work as a sustainability professional, their roles, and daily work practices. To practice, this thesis contributes by providing knowledge that may support the future development of sustainability roles.