Immunoassays were first used in the 1950s for the identification and quantification of analytes, with the ELISA (enzyme-linked immunosorbent assay) first described in 1971 by Engval and Perlmann. Since then, the range of applications has constantly expanded, and immunoassays are now indispensable in different fields from clinical research to drug discovery. In addition to the classic colorimetric ELISA, other detection technologies have been established. These not only enable a higher sensitivity, but also expand the range of applications of the assays. This chapter introduces some established microplate reader-based systems that can be used as alternatives to classic methods for the identification and quantification of analytes and for the investigation of possible interactions.