A mathematical billiard is a system describing the inertial motion of a point mass inside a domain, with elastic reflections at the boundary. Despite their apparently simple (local) dynamics, their qualitative dynamical properties are extremely non-local, and there is a tight intertwinement between its dynamics and the shape of the domain. All of this translates into several intriguing rigidity phenomena, which are at the basis of several unanswered questions and conjectures.