Clinicians should note that there is considerable variability in the reliabilities of the index and subtest scores derived from the third editions of the Wechsler Adult Intelligence Scale (WAIS-III) and the Wechsler Memory Scale (WMS-III). The purpose of this article is to review these reliabilites and to illustrate how they can be used to interpret change in patients' performances from test to retest. The WAIS-III IQ and Index scores are consistently the most reliable scores, in terms of both internal consistency and test–retest reliability. The most internally consistent WAIS-III subtests are Vocabulary, Information, Digit Span, Matrix Reasoning, and Arithmetic. Information and Vocabulary have the highest test–retest reliability. On the WMS-III, the Auditory Immediate Index, Immediate Memory Index, Auditory Delayed Index, and General Memory Index are the most reliable, in terms of both internal consistency and test–retest reliability. The Logical Memory I and Verbal Paired Associates I subtests are the most reliable. Data from three clinical groups (i.e., Alzheimer's disease, chronic alcohol abuse, and schizophrenia) were extracted from the Technical Manual [Psychological Corporation (1997). WAIS-III/WMS-III Technical Manual. San Antonio: Harcourt Brace] for the purpose of calculating reliable change estimates. A table of confidence intervals for test–retest measurement error is provided to help the clinician determine if patients have reliably improved or deteriorated on follow-up testing.