Sustainability assessment has emerged as a key decision-support process in development cooperation in response to the growing acknowledgement of the impacts of global change. This paper aims at conceptualizing the effectiveness of sustainability assessment as applied in development cooperation, by focusing on the sustainability assessment practice by actors of the official Belgian Development Cooperation. The conceptualization of the effectiveness of sustainability assessment is synthesized in a set of issues and concerns, based on semi-structured interviews. The paper highlights the specificity of sustainability assessment in the development cooperation sector (e.g., through the cultural and discursive compatibility dimensions of assessment in a North-South context). Effectiveness is inherently linked to the expected functions of sustainability assessment in the decision-making process, which include fostering organizational change, shaping contextually adapted framings of sustainability and operationalizing the sustainability transition. These findings highlight the relevance of a discourse-sensitive approach to sustainability assessment if one is to strengthen its credibility and legitimacy.