An overview of the applications of luminescent transition metal complexes to sensor technology is presented. In addition to general considerations, a number of specific systems are discussed including oxygen, pH, carbon dioxide, temperature, and immunoassay sensors. Applications of the complexes to molecular probes are also covered including chirality and conformational proves, water exposure sensors, and dynamic probes. Problem areas such as photostability, excitability, immobilization chemistry, the existence multiple responses in the same molecule, and substrate-support interactions are discussed. Also covered is the very difficulty problem of unambiguously interpreting the complex kinetic and chemical behavior of real sensor systems.