This article introduces a special issue of the Journal of Rational-Emotive & Cognitive-Behavior Therapy on assessment and diagnostic issues in REBT. Contributions to the special issue include reviews of (a) the psychometric properties of irrational beliefs measures and (b) the potential utility for REBT research and practice of behavioral measures of distress tolerance; (c) a primary study of the diagnostic and functional significance of high trait anger, with implications for classification and REBT treatment of anger disorders; and (d) a closing commentary by John Malouff on themes raised in the earlier papers.