Sensitive immunoassays were developed in the 1950s and 1960s using radioactive isotopes. The application of enzymes as labels (1 ,2 ) in the late 1960s increased the potential sensitivity and safety of immunoassays. The widespread popularity of enzyme-linked immunosorbent assays (ELISAs) owes much to the use of microtiter plates (3 ). It was probably the adoption of microtiter plates that provided ELISA with its greatest advantag-convenience. However, the convenience of plastic microtiter plates carries with it a limitation, the ease with which reagents bind to plastic and the way in which they perform once bound. In this chapter, I will review ELISA procedures and discuss their advantages and disadvantages.