Measurement error is a common theme in classical measurement models used in testing and assessment. In classical measurement models, the definition of measurement error and the subsequent reliability coefficients differ on the basis of the test administration design. Internal consistency reliability specifies error due primarily to poor item sampling. Rater reliability indicates error due to inconsistency among raters. For estimates of test-retest reliability, error is attributed mainly to changes over time. In alternate-forms reliability, error is assumed to be due largely to variation between samples of items on test forms. Rasch models can also compute reliability estimates of scores under different test situations. The authors therefore present the Rasch perspective on calculating reliability (measurement error) and present Rasch measurement model programs to compute the various reliability estimates. (Contains 5 tables and 1 note.)