Purpose: This research intends to shed a light in the practice of project portfolio management in the non-traditional – although project oriented – aid sector. The research aim is to study the decision-making structures supporting the prioritization of projectsand/or programmes in multilateral organizations, which play a determinant role in the development aid sector. Research Methodology: Through an in-depth and-holistic case study, the empiricalresearch investigated how the UN coordination practitioners perceived the role of thecontext in the implementation of the Delivery as One Approach, which comprehends aset of standards and procedures (SOPs) supporting the management of multiple UN entities at the country level, to enhance effectiveness, efficiency, coherence and impact. A total of 9 semi-structured interviews were conducted with current and former employees in the UN resident coordination office in a range of countries in Africa, LatinAmerica and the Middle East. From the collection of qualitative data, the researchers wereable to grasp the nuances of the data set through the elaboration of templates, which based the further discussion and conclusions of the work. Research Findings: The empirical findings confirmed the relevance of a number of constructs identified in the theoretical framework, defining how the context influences the decision making that takes place in the prioritization of programmes in the development aid sector. Specifically, the results highlight the relevance of the governancestructure, the bounded rationality of decision makers, specific characteristics of the decision, country peculiarities, and the different sources of uncertainty. Moreover, the relationships between these factors were highlighted through a relationship network diagram that clearly identifies the complex interrelations between these factors and theirsub-themes. Research Delimitations and Limitations: The delimitations in this study are characterized as the choices made by the researchers on the parameters considered and mentioned, setting the boundaries for the investigation. From a methodological standpoint, by using the single case study method, the findings and conclusion of the present research applies majorly to the organization studied. Originality / Value: This research advances the portfolio management literature on the field of international development aid and expands the understanding of how the aspects of this unique environment influences the decision making of assigning priorities to projects and programmes. Furthermore, the research draws attention to the different sources of uncertainties originating from the context, inherent of these types oforganizations.