Peacebuilding endeavours have long been sabotaged by flawed assumptions that see the world as if problems can be identified and solved in relative isolation. Specifically, mainstream peacebuilding unwittingly relies on the notion of ceteris paribus, which enables a focus on certain aspects without a concern for everything else—which remains constant, frozen. But societies are not made of independent parts, rather a constantly changing web of relationships. We therefore need analytical tools that are compatible with the fluid nature of peacebuilding environments; we need to ‘thaw’ ceteris paribus, replacing it with a Complexity lens. After contrasting the mainstream and Complexity lenses, the chapter suggests ways to adapt popular evaluation tools and concludes with ideas about how to mainstream Complexity.