In September 1971, Swiss scientists Eva Engvall and Peter Perlmann published a communication in the journal Immunochemistry on enzyme-linked immunosorbent assay (ELISA) as a quantitative assay of immunoglobulin G. They have modified radioimmunoassay (RIA) by replacing the radioactive iodine with enzymes and used in determining the levels of IgG in rabbit serum. Fifty years after, ELISA is still the most commonly used technique in measuring peptides, proteins, antibodies, antigens, and hormones in various samples. This plate-based assay is a very versatile laboratory technique and offers many advantages to laboratory scientists and researchers such as generating quick results, simple protocols, high sensitivity, being easily automated and being convenient to perform. Its application in the diagnosis of different infectious diseases has shown the unquestionable importance of this technique even in the modern world. In this book, the usefulness of ELISA in the control of different parasitic diseases including schistosomiasis, babesiosis, and malaria will be discussed. There is also a chapter featuring an extensive discussion on the use of ELISA in functional food development and regulation. And finally, the last two chapters will present advanced techniques inspired by ELISA including a rapid immunoassay for detecting paralytic shellfish poisoning toxins and the enzyme-linked aptamer assay. May this book provide significant information regarding the current advances and applications of ELISA and possibly inspire its use in another 50 years.