This primer is an introduction to item response theory (also called item characteristic curve theory, or latent trait theory) as it is used most commonly--for scoring multiple choice achievement or aptitude tests. Written for the testing practitioner with minimum training in statistics and psychometrics, it presents and illustrates the basic mathematical concepts needed to understand the theory. Then, building upon those concepts, it develops the basic concepts of item response theory: item parameters, item response function, test characteristic curve, item information functions, test information curve, relative efficiency curve, and score information curve. The maximum likelihood and Bayesian modal estimates of ability are described with illustrative examples. After a discussion of assumptions and available computer programs, some practical applications are presented, i.e. equating scales, tailored testing, item cultural bias, and setting pass-fail cut-offs. (Author/CP)