This chapter covers negative spillover effects. Unintended negative spillover effects materialize when an external intervention negatively affects thematic or geographic areas or institutions outside of the intervention area. This chapter covers four kinds of negative spillover effects: negative institutional spillover effects (‘brain drain’), negative thematic spillover effects (‘thematic crowding out’), negative geographic spillover effects (‘leakage’), and fungibility. Negative institutional spillover effects (‘brain drain’) occur when the capacities of more permanent local structures (e.g. a government agency) are weakened as international agencies hire the most talented staff without adequately compensating the local structure. Negative thematic spillover effects (‘thematic crowding out’) occur when an upsurge of international attention for a specific (sub) theme pushes out other thematic areas that also require attention. Negative geographic spillover effects (‘leakage’) occur when the international intervention solves a problem in one place, but this or another problem worsens elsewhere. Fungibility occurs when aid resources do not pay for the item it is accounted for but for the marginal expenditure it makes possible.