Sanctions and mediation are often applied simultaneously by the UN, but there has been little systematic exploration of their interrelationships. Drawing on research from the Sanctions and Mediation Project (SMP), both complications and complementarities can be identified. Sanctions can complicate mediation by fostering exclusion, emboldening nonsanctioned parties, closing mediation space, undercutting mediator impartiality, and forcing premature agreements. At the same time, sanctions can complement mediation by deterring spoilers, breaking stalemates, incentivizing cooperation, modifying cost-benefit calculations, ensuring broad participation in talks, and facilitating the onset of talks. The conditions under which complementarity can be enhanced include UN Security Council unity, focus and coherence of mandates, and regional cooperation. This article concludes with policy recommendations for different institutional actors and some suggestions for future research.