Hydrogen sulfide (H₂S) is a small molecule that plays crucial roles in various biological processes, acting as a "gasotransmitter" and influencing cellular functions. The development of fluorescent probes for H₂S and related reactive sulfur species (RSS) has been a significant advancement in understanding these molecules' roles. This review focuses on activity-based fluorescent probes that couple specific H₂S-mediated chemical reactions with a fluorescent output. These probes provide high spatiotemporal resolution and have revolutionized the study of H₂S in complex biological systems. The review covers different types of activity-based probes, including those that reduce oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. Each probe type is activated by specific RSS, such as persulfides and polysulfides. The chemical mechanisms behind these activations are detailed, highlighting the unique properties of each probe. Key advances in the development of these probes are discussed, emphasizing their selectivity and compatibility with cellular systems. The review also addresses the challenges in developing selective probes, particularly the interference from other thiols and nucleophilic sulfur oxides. Examples of probes that have been successfully used in live-cell imaging and in vivo studies are provided, demonstrating their utility in investigating H₂S's multifaceted roles in biology. The scope of the review includes both small molecule-based approaches and complementary fluorescence microscopy techniques. It aims to highlight the breadth and depth of activity-based H₂S and RSS sensing, providing insights into the chemistry and applications of these probes.Hydrogen sulfide (H₂S) is a small molecule that plays crucial roles in various biological processes, acting as a "gasotransmitter" and influencing cellular functions. The development of fluorescent probes for H₂S and related reactive sulfur species (RSS) has been a significant advancement in understanding these molecules' roles. This review focuses on activity-based fluorescent probes that couple specific H₂S-mediated chemical reactions with a fluorescent output. These probes provide high spatiotemporal resolution and have revolutionized the study of H₂S in complex biological systems. The review covers different types of activity-based probes, including those that reduce oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. Each probe type is activated by specific RSS, such as persulfides and polysulfides. The chemical mechanisms behind these activations are detailed, highlighting the unique properties of each probe. Key advances in the development of these probes are discussed, emphasizing their selectivity and compatibility with cellular systems. The review also addresses the challenges in developing selective probes, particularly the interference from other thiols and nucleophilic sulfur oxides. Examples of probes that have been successfully used in live-cell imaging and in vivo studies are provided, demonstrating their utility in investigating H₂S's multifaceted roles in biology. The scope of the review includes both small molecule-based approaches and complementary fluorescence microscopy techniques. It aims to highlight the breadth and depth of activity-based H₂S and RSS sensing, providing insights into the chemistry and applications of these probes.