Can triangular co-operation make aid more effective? Judging by recent international declarations, governments think it can. The underlying assumption is that better results are achieved when Southern partners and “traditional” donors (i.e. those that gather in the OECD Development Assistance Committee) join forces through triangular co-operation. This article addresses what triangular co-operation is, which countries are involved in it and why; examines the claimed benefits of triangular co-operation vis-à-vis bilateral forms of co-operation; identifies some challenges in rendering triangular co-operation an effective mode of development co-operation; and provides recommendations on how these challenges can be met.