► Hydrogen sulfide (H S) can mediate physiology and disease pathology in mammalian systems. ► The development of molecular probes for H S detection allows H S to be studied in intact cells, tissues, or whole organisms. ► Current probes employ a variety of reaction-based strategies to achieve selectivity for H S over other biological thiols. Hydrogen sulfide (H S) has long been recognized as a toxic molecule in biological systems. However, emerging studies now link controlled fluxes of this reactive sulfur species to cellular regulation and signaling events akin to other small molecule messengers, such as nitric oxide, hydrogen peroxide, and carbon monoxide. Progress in the development of fluorescent small-molecule indicators with high selectivity for hydrogen sulfide offers a promising approach for studying its production, trafficking, and downstream physiological and/or pathological effects.