There is broad support worldwide for the concept of sustainable development and the integration of its three pillars: economicdevelopment, environmental protection and social development. Nevertheless, previous research shows substantial difficul-ties associated with fully incorporating and operationalising social sustainability features in various sectors. The presentarticle aims to explore further the reasons why incorporation of social sustainability aspects appears to pose a challenge.The article has a twofold explorative aim. First, the aim is to identify opportunities/benefits or difficulties/detriments thatemerge when actors try to incorporate social aspects into sustainability projects. Second, the article probes for explanationsfor the observed challenges. This is done by referring to a case study examining how the Forest Stewardship Council (FSC)has attempted to incorporate social sustainability goals, principles and criteria. Using qualitative interviews, FSC-relateddocuments, participant observation, as well as previous research, the article examines the successes and challenges asso-ciated with including social sustainability features in the standards and certification process. Observed achievements anddifficulties are highlighted in relation to four general aspects: (1) improvement of substantive social sustainability goals; (2)local organisation, empowerment and employment; (3) communication; and (4) small-scale and community-based forestry.The article suggests and analyses eight reasons for these challenges, which relate to discursive, structural or organisationalaspects. The findings presented here may also be useful in attempts to understand other similar integrative transnationaland/or local sustainability projects