This chapter focuses on a paradigmatic topological crystalline insulator: SnTe. This symmetry-protected topological phase is defined, with an emphasis on SnTe. The effect of Sb impurities on the Dirac-like topological surface states of SnTe is studied using several simulation techniques and continuum models. The conditions for the robustness of the surface states, as well as the interaction between impurity and surface states, are analysed depending on the position and number of impurities.