This chapter covers the backlash effect. Unintended backlash effects occur when an external action leads to an opposite reaction that affects achieving the intended effect. The backlash effect is by definition a macro-level effect, as reactions that are opposite to the expectation at the individual level fall under the heading of behavioural effects. This chapter covers two kinds of backlash effects: anti-aid backlash and anti-foreign values backlash. Anti-aid backlash occurs when an external intervention is met with suspicion about the ulterior motives of the aid actor. This suspicion often has historical origins. If implemented clumsily, the intervention itself can fire up latent discontent about foreign support, and local leaders can stir up discontent if it serves them. The backlash against aid reduces the effectiveness and sustainability of the intervention. Anti-foreign values backlash occurs when values that external actors promote clash with the mainstream values present in a society. The anti-foreign values backlash shares many of the characteristics of the anti-aid backlash, but finds its origins in the cultural distance between aid actors and societies where they operate rather than discontent about ulterior motives.