Each new ability to visualize analyte concentrations in real-time portends the fabrication of new sensors. Potential applications include fiber optic-based remoting sensing of solution species and the monitoring of concentrations within living cells. A fluorescent chemosensor is a compound of abiotic origin that complexes to an analyte reversibly with concomitant fluorescence signal transduction transduction. Such chemosensors have been used for fluorimetric metal ion determination for over 100 years. The advent of ligand engineering has introduced a more systematic approach to the design of chemosensors with new selectivities and signal transduction schemes. The technological driving force to achieve useful chemosensors will stimulate investigation of new topics in molecule recognition, fluorescent signal transduction, and their intersection.