abstract: It is usually not possible to complete the billiard dynamics on a table with corners so as to include trajectories that are reflected in corners in a continuous way. Moreover, even if the boundary of a convex body is three times differentiable, discontinuity phenomena can occur. The geodesic flow on the boundary may not match up continuously with the billiard flow inside. In the talk I will discuss conditions for a continuous billiard flow completion to exist and explain the underlying methods from comparison geometry. On the way I will mention other applications of metric geometry to billiards.