By relating observed and modelled behaviour, conformance checking unleashes the full power of process mining. Techniques from this discipline enable the analysis of the quality of a process model discovered from event data, the identification of potential deviations, and the projection of real traces onto process models. This way, the insights gained from the available event data can be transferred to a richer conceptual level, amenable for a human interpretation. The aforementioned functionalities are grounded on the use of conformance checking artefacts that explicit the relation between observed and modelled behaviour. This chapter describes these artefacts, and builds upon them to gain evidence-based insights on the processes of an organization. Moreover, we overview the applications of conformance checking and propose a general framework that incorporates these applications. Finally, milestones and challenges of the field are outlined.