Many Organisation for Economic Cooperation and Development (OECD) countries recruit internationally skilled workers for their health, education, or public administration sectors and the subsequent emigration of such workers can cause critical shortages in developing countries, even as these countries receive substantial aid from those same OECD countries. Similar inconsistencies emerge from the interaction of aid and trade policies, when trade-distorting subsidies or barriers to market access undermine the effects of aid policy, most notably for trade capacity building. Policy coherence for development can be defined as the pursuit of development objectives through the systematic promotion of mutually reinforcing policy actions on the part of both OECD and developing countries. This report surveys some of the core issues in the Policy Coherence for Development agenda, asking, in particular, which is the best way to think about policy interactions? The authors focus on the distinction between policies and flows and explore the evidence on the directions of causality across both flows and policies. The report also highlights the difference between complementarity and substitutability of policies by looking more closely at three types of policy interactions: (1) trade/aid; (2) trade/migration; and (3) aid/migration, and closes with implications of relevance for policy makers. (Contains 24 notes, 2 boxes and 2 figures.)