Young children with autism spectrum disorder are increasingly placed in general education classrooms, particularly in the early grades (kindergarten through grade 3). Estimates suggest at least 40% of general education teachers have these children in their classrooms, and many report feeling ill-equipped to support them, primarily due to lack of training and professional development. One construct that is key to the success of children with ASD in general education classrooms is their relationship with the teacher. Indeed, previous literature has linked high-quality student-teacher relationships (STRs) to more positive outcomes including fewer behavior problems, better academic performance, and social-emotional adjustment in the early elementary grades. Yet students with autism spectrum disorder are at heightened risk of developing poor-quality STRs. Thus, there is need for socially valid teacher training that, in addition to increasing autism-related knowledge, emphasizes the development of warm and less conflictful STRs. In addition, the evaluation of most teacher training programs has focused exclusively on effectiveness (i.e., randomized-control trials), without a thorough examination of social validity. The purpose of this study was to examine the social validity of the "Smooth Sailing" program through a concurrent mixed methods approach. Results indicated that (1) teacher knowledge of ASD increased substantially between pre- and post-program, and (2) teachers found the "Smooth Sailing" program socially valid. The mixed-methods approach provided nuanced information about the acceptability, feasibility, perceived effectiveness, and contextual fit of the program. Implications for educational practice and implementation science, as well as suggestions for future 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.]