Presheaves are widely used as models for concurrency. The aim of the talk is to give an introduction to presheaves and the associated notion of bisimulation called open maps. Mathematically, presheaves are attractive because they are simple (presheaves are just set-valued functors), cover many interesting examples (e.g. unary algebras, transition systems, simplicial sets), and have a lot of structure (for example, categories of presheaves are toposes). Open maps can be described by weak factorisation systems.