The present study used a multiple probe across behaviors design, with an embedded alternating treatments design to compare the effects of two modified communication systems, the Picture Exchange Communication System (PECS) and a speech generating device, on requesting for a child with multiple disabilities, including autism and visual impairments. Both modalities were taught across the first three phases of PECS instruction (simple request, distance and persistence, and discrimination). During baseline the participant demonstrated little to no requesting across all three phases for either modality. During intervention requesting increased to proficient levels across all three phases for both modalities. Modified PECS, however, yielded noticeably faster results in the distance and persistence phase. Social validity and preference for each modality were also assessed. Both communication systems were found to be acceptable. Both the parent and participant showed preference for the modified PECS. The importance of social validity and modality preference is 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.]