This chapter discusses the emergence of strong optical nonlinearities from Rydberg interactions. Key effects such as the single-photon Kerr nonlinearity, electromagnetically induced transparency (EIT), and their quantum extensions are developed. The role of the cavity in enhancing blockade and enabling cavity-Rydberg polaritons is analyzed in depth. Particular attention is given to the formation of dark-state polaritons and their utility for realizing quantum nonlinear media, paving the way toward single-photon switches and quantum light sources.