Topological photonics has been raising significant interest in the past years, due to the exciting fundamental advances it has unveiled in the way we can control light with engineered materials, and for the broad opportunities it has opened for applications. This field of research has taken inspiration from recent advances in condensed matter physics highlighting the important connections between the topological features of the band diagram of an infinite medium and the electronic response of a finite sample of such material. In particular, topological insulators are a class of insulating materials whose bandgap has nontrivial topological features, based on which robust conduction properties are expected across the entire bandgap at the boundaries of any finite sample of such material. Their inherent robustness is rooted into these topological features, which are not affected by continuous perturbations and disorder, and by the underlying symmetry protection that drives the nontrivial topology.