Individuals who are dually diagnosed with Down Syndrome (DS) and autism spectrum disorders (ASD) frequently engage in vocal stereotypy that can interfere with learning, impede socially appropriate behavior, and disrupt others. Response interruption and redirection (RIRD) is an intervention that blocks and redirects stereotypy to reduce behavior. Studies on RIRD, while varied in their overall approach, have overwhelmingly been shown to be effective in clinical settings for children with ASD. There are few studies that have evaluated this procedure in more natural settings and with other diagnoses. Given that ASD is prevalent in those with DS and that stereotypy is more common in those who present with this dual diagnosis, this study evaluated RIRD as a potential treatment for this population. Using a single subject research design, the participant's vocal stereotypy was measured during baseline and intervention phases. Vocal stereotypy reduced during RIRD treatment. A social validity questionnaire was given to caregivers at the conclusion of the study, and the intervention was rated positively, showing high levels of satisfaction. The implications of these findings and limitations of the research are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]