Immunoassays can be characterized as quantitative analytical methods applied for measuring biologically important compounds/organisms using antibodies as specific analytical reagents. The ELISA procedure completely differs from the classical serological techniques of immunoprecipitation, immunodiffusion, and agglutination wherein a considerable degree of polymerization is needed before the threshold of visual detection is reached. Generally microtiter plates or strips of polystyrene are used as solid supports for conducting ELISA test. However, other plastic solid phases have also been tried for enzyme immunosorbent assays. It is expected that the user is aware of the various forms of ELISA and is able to rightly select an appropriate procedure before starting the test. There has been a substantial impact of ELISA in the large scale diagnosis of diseases. ELISA has revolutionized the diagnosis for assessing disease for certification purposes and for control through quarantine or eradication procedures. Indexing for seed-borne infections has been greatly successful particularly for the viruses and bacteria.