Fluorescent probes and fluorometric methods have gained increasing interest in the field of clinical immunology, not only as one additional alternative to radioimmunoassays, but also in producing cheap, stable, and safe reagents and rapid and sensitive assays. One of the main goals has been the development of homogeneous assays: assays based on fluorescence polarization, fluorescence quenching, excitation transfer, or enzymically releasable probes are widely applied, especially in drug monitoring. The development of suitable solid-phase separation techniques has facilitated utilization of fluorescence in heterogeneous assays, which in general have wider applications, from proteins and viruses to small haptens. Lately new alternative fluorescent probes and methods have been introduced. For example, the use of fluorescent phycobiliproteins or porphyrin derivatives with long-wavelength emission and large Stokes shift or, in particular, the rare earth chelates with unique fluorescent properties well suited to time-resolved measurement have opened new possibilities towards more sensitive immunoassays.