The Rasch model of test theory is described in general terms, compared with latent trait theory, and shown to have interesting applications for the measurement of affective as well as cognitive traits. Three assumption of the Rasch model are stated to support the conclusion that calibration of the items and tests is independent of the examinee sample, and that measurement of the trait is independent of the items used, a condition called specific objectivity. Item characteristic curves are defined and related to item slope and item discrimination. The procedure for testing the fit of items to the model is discussed. The test of whether an examinee's responses fit the model may be used to indicate whether a particular score is valid. The test of item fit may be used to investigate the nature of affective variables. A calibrated item pool is useful for repeated testing and for using as a standard for new items. The Rasch model is useful for relating a series of tests to a single longitudinal scale. Thirty bibliographical references are appended. (CTM)