This brief provides a summary of background research for future aid-related EPAR projects. We first review prominent measures of aid, examining the definition and scope of Official Development Assistance (ODA) as well as common criticisms and alternatives to this measurement. We also provide a summary of current research on bilateral and multilateral aid allocation trends. The aid allocation literature broadly concludes that donor countries target aid based on both the needs of recipients and on strategic interests, but that aid allocation criteria differ by donor and by type of aid. Finally, we summarize current aid effectiveness literature and key challenges in exploring the impact of aid. A number of challenges in determining the effectiveness of aid were common in the literature, including the micro-macro paradox, difficulties in identifying causal mechanisms and direction of causality, and data limitations.