The enzyme-linked immunospot (ELISPOT) assay took its design concept from traditional ELISA techniques and evolved over the years from a method for detecting antibodies secreted from B cells to a method for detecting cytokines or other soluble mediators secreted from a variety of different cell types. The ELISPOT assay allows the quantitative measurement of frequency of cytokine secreting cells at the single cell level directly ex vivo without elaborate in vitro expansion or manipulation of cell populations. The function of cells can be inferred from the pattern of cytokines secreted by cells in response to diverse antigenic stimuli and thus the ELISPOT assay has become a powerful method for monitoring immune responses in health and disease. The ELISPOT assay like the ELISA assay is relatively easy to perform and it has the promise of robustness, reliability, and reproducibility of performance for use as a diagnostic tool. The history, applications, validation process, and future challenges of the ELISPOT assay are discussed in this chapter.