The authors present an elementary and exceptionally lucid introduction to issues in measurement theory. They define and discuss validity and reliability; proceed to a discussion of three basic types of validity, including criterion, content, and construct validity; present an introductory discussion of classical test theory, with an emphasis on parallel measures; and present a clear discussion of four methods of reliability estimation, including the test-retest, alternative form, split-half, and internal consistency methods of reliability assessment. The text is concluded with a discussion of the use of reliability assessment for purposes of correcting bivariate correlations for attenuation due to random measurement error.