Immunoassay is a powerful technique for quantitative and qualitative analysis of a wide range of analytes. These include peptides, proteins, simple organic molecules, environmental pollutants, and pharmaceuticals. Immunoassay has been used since the mid-1950s as an analytical tool and has developed during that time into a mature analytical methodology that is, however, still developing. Its advantages are sensitivity, throughput, and selectivity. Immunoassay is capable of very low limits of quantication and allows a very high sample throughput. Selectivity is usually good, but is dependent, however, on the analyte in question; this can limit its usefulness when there are very similar, structurally related molecules present in the matrix. Although highly sensitive, this technique has not been widely used for routine bioanalysis of small molecules in the pharmaceutical industry for a number of reasons, but is now enjoying a resurgence, mainly because it is one of the very few reliable techniques available for rapid and sensitive analysis of large pharmacologically active molecules, such as monoclonal antibodies and other proteins that are now becoming important. A further rapidly developing eld in which immunoassays are crucial is that of biomarker analysis. Many pharmaceutical companies now routinely develop biomarker assays as a means of determining the efcacy of new treatments, or to ascertain whether a particular target mechanism is of importance to disease and disease modication. Biomarkers for such systems are very often proteins, and the only easily accessible analytical method is immunoassay for such markers.