Eye movement desensitization and reprocessing (EMDR), though controversial, is increasingly utilized for treatment of posttraumatic stress disorder (PTSD). This article reviews the debate concerning efficacy and concludes that the evidence, though not definitive, supports EMDR's positive treatment effects. The authors argue that EMDR is a therapeutic intervention different from exposure. The authors set forth three interrelated hypotheses to explain EMDR's therapeutic mechanism: bilateral hemispheric activation, normalized brain activation patterns, and activation/desensitization of emotion/arousal; avoidant/constricted attention is disrupted, allowing normalizing processes to occur. Lowered arousal then leads to a resumption of more adaptive cognitive processing. Some predictions to test this model are presented. Copyright © 2002 John Wiley & Sons, Ltd.