Much of the work in data handling for enzyme-immunoassay has done before enzyme-immunoassay techniques became a significant laboratory tool. The efforts of R.F. Ekins, D. Rodbard, and others working in radioimmunoassay have provided a strong foundation, much of which can be borrowed directly, for data analysis and quality assurance in enzyme-immunoassay. This chapter discusses the two groups of people. Those who wish an introduction to enzyme-immunoassay data analysis may want to skim the formulas and concentrate on the descriptions of models and quality assurance concepts. Those who wish to apply some of the results of the past ten years to their own work will find explicit descriptions of simple models and their application, and a guide to some of the more sophisticated mathematical techniques. For homogeneous enzyme-immunoassay and heterogeneous enzyme-immunoassay using enzyme-labeled antigen, a rather simple model for the competition reaction can derived under the assumption that there is only one binding site on both the antibody and antigen-enzyme conjugate.