This is a multi-method study to develop and validate an instrument to measure teachers' self-efficacy for teaching students with autism spectrum disorder (ASD) in inclusive early childhood classrooms, "Teacher Self-efficacy for Teaching Students with ASD Inclusive Classrooms Scale": TSE-ASDI. I conducted literature and expert reviews as well as cognitive pre-testing with my target populations of pre-service and in-service early childhood teachers. I conducted a quantitative study using exploratory factor analysis, reliability analyses, correlational analyses, and by comparing mean differences in scores when grouped by teaching status, special education preparation and experience with individuals with ASD. My measure development process provided evidence for validity based on test content, response process, internal structure of the instrument as well as evidence based on relations to other variables. The result of this process was a highly reliable, unidimensional, 16-item scale to measure the construct of teaching students with ASD in inclusive early childhood classrooms. Based on these findings, this investigation has implications for research and practice. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]