This article discusses the rationale of an item response theory (IRT) model, based on Thurstone scaling, for personality measurement and suggests procedures for (a) estimating the location and slope parameters of the person response curve (PRC), (b) assessing the precision of the estimates, and (c) assessing the model-data fit. The relations between the PRC parameters and the l0 and lz person-fit measures are also discussed. The procedures described and the relations discussed are illustrated using empirical data.