The effectiveness of official development aid is the subject of heated debate. This column argues that aid affects recipient economies in extremely complex ways and through multiple and changing channels. Moreover, this is a two-way relationship – realities in recipient countries affect the actions of aid agencies. This relationship is so intricate and time-dependent that it is not amenable to being captured by cross-country or panel regressions. Even sophisticated specifications with multiple breakpoints and nonlinearities are unlikely to explain the inner workings of the aid–performance connection.