Fluorescent chemosensors, which have been widely applied in biology, physiology, pharmacology, and environmental sciences, have a history of about 150 years. This tutorial review provides an overview of the development of fluorescent chemosensors, highlighting pioneering works from around 40 groups. The review covers the basic principles of chemosensor design, common photo-physical mechanisms for analyte detection, and the challenges and future research directions in the field. Key learning points include the definition of fluorescent chemosensors, the first reported chemosensor for cations, and the importance of cations in biological processes. The review also discusses the development of chemosensors for anions, small neutral molecules, and biomacromolecules, emphasizing their applications in various biotechnologies.Fluorescent chemosensors, which have been widely applied in biology, physiology, pharmacology, and environmental sciences, have a history of about 150 years. This tutorial review provides an overview of the development of fluorescent chemosensors, highlighting pioneering works from around 40 groups. The review covers the basic principles of chemosensor design, common photo-physical mechanisms for analyte detection, and the challenges and future research directions in the field. Key learning points include the definition of fluorescent chemosensors, the first reported chemosensor for cations, and the importance of cations in biological processes. The review also discusses the development of chemosensors for anions, small neutral molecules, and biomacromolecules, emphasizing their applications in various biotechnologies.