This collection of David H. Barlow’s key papers are a testimony to the collaborative research that he engendered and directed with associates who now stand with him at the forefront of experimental psychopathology research and in the treatment of anxiety and related disorders. His research on the nature of anxiety and mood disorders resulted in new conceptualizations of etiology and classification. This research led new treatments for anxiety and related emotional disorders, most notably a new transdiagnostic psychological approach that has been positively evaluated and widely accepted. Clinical psychology will benefit from this collection of papers with connecting commentary.