In enzyme-linked immunosorbent assay (ELISA), a typical protocol specifies that a certain number of wells in a plate be devoted to analysis of known concentrations for calibration. The number of calibration wells and the known concentrations used affect the precision of the determinations of the unknowns. as does the choice of the number of replicates of each unknown. Similar design decisions must be made for other forms of immunoassay. This chapter shows how these factors can be chosen to give the maximum system precision. Other statistical aspects of immunoassay are discussed.