2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/79834The properties of optical solitons in periodic nonlinear media are reviewed. The emphasis is on solitons in periodically refractive media (Bragg gratings) incorporating a periodic set of thin layers of two-level systems resonantly interacting with the field. Such media support a variety of bright and dark `gap solitons' propagating in the band gaps of the Bragg gratings, as well as their multi - dimensional analogs (light bullets). These novel gap solitons differ substantially from their counterparts in periodic media with either cubic or quadratic off-resonant nonlinearities.Review paper submitted to `Progress in Optics'. 48 pages, 16 figuresPattern Formation and SolitonsOpticstext