Inaccuracies in administration and scoring can potentially compromise the validity of any standardized psychosocial measure. The threat is particularly pertinent to methods involving behavioral observation, a category that includes many intelligence tests, neuropsychological measures, personality assessment instruments, and diagnostic procedures. Despite evidence and conjecture that errors in testing procedure are common for at least some of these measures and that these errors are often severe enough to influence interpretation, the topic has received relatively little attention. In particular, the absence of any safeguard against inaccurate test use in clinical situations can put the respondent at risk and violates ethical standards for the use of tests. In this article, I review some issues surrounding accuracy in testing procedures, including a discussion of what is known about the problem, an evaluation of several approaches to improving testing practices, and a review of recommendations for the statistical evaluation of rater accuracy. In this article, I use the Rorschach Comprehensive System (Exner, 1993) to demonstrate the concepts discussed.