This review discusses reaction-based small-molecule fluorescent probes for chemoselective bioimaging. These probes use selective, bioorthogonal chemistries to detect specific analytes in cells and complex biological specimens. They offer powerful tools to study reactive chemical species in their native environments with minimal disruption. The review highlights design principles for effective chemical tools and gaps in future research. It covers various reaction-based approaches, including oxidative and reductive cleavage reactions, nucleophilic reactions, and metal-mediated reactions, for detecting biologically important molecules such as reactive oxygen, nitrogen, and sulfur species, as well as metal ions. Examples include probes for nitric oxide, hydrogen peroxide, and hypochlorous acid, which have been used to study their roles in various biological processes. The review also discusses the development of fluorescent probes for metal ions, such as mercury and copper, using strategies like Lewis acid hydrolysis and organometallic reactions. The importance of bioorthogonal reactions and the need for reactions that are compatible with living systems are emphasized. The review concludes that reaction-based bioimaging has significant potential for advancing our understanding of biological processes and offers exciting future prospects in the field of bioimaging.This review discusses reaction-based small-molecule fluorescent probes for chemoselective bioimaging. These probes use selective, bioorthogonal chemistries to detect specific analytes in cells and complex biological specimens. They offer powerful tools to study reactive chemical species in their native environments with minimal disruption. The review highlights design principles for effective chemical tools and gaps in future research. It covers various reaction-based approaches, including oxidative and reductive cleavage reactions, nucleophilic reactions, and metal-mediated reactions, for detecting biologically important molecules such as reactive oxygen, nitrogen, and sulfur species, as well as metal ions. Examples include probes for nitric oxide, hydrogen peroxide, and hypochlorous acid, which have been used to study their roles in various biological processes. The review also discusses the development of fluorescent probes for metal ions, such as mercury and copper, using strategies like Lewis acid hydrolysis and organometallic reactions. The importance of bioorthogonal reactions and the need for reactions that are compatible with living systems are emphasized. The review concludes that reaction-based bioimaging has significant potential for advancing our understanding of biological processes and offers exciting future prospects in the field of bioimaging.