This applied dissertation was designed to improve the effectiveness of professional educators in using the methodology of functional behavior assessment (FBA) and function assessment-based intervention (FABI) to reduce challenging behaviors exhibited by students with Down syndrome (DS), or a dual diagnosis of Down syndrome and autism spectrum disorder (DS-ASD). Educators often rely on traditional classroom management methods which curtail disruptive behaviors without determining the reason why the behavior is occurring. The study examined the effects of utilizing a behavioral skills training program of instruction, modeling, rehearsal, and feedback in the methodology of FBA and FABI. The research was conducted using a single subject research design of multiple probes across participants to train three special educators working with students with DS or DS-ASD. All participants increased their ability to identify the function of the behavior and select appropriate interventions as measured by an increase in knowledge and percent correct of functionally appropriate interventions selected. There was a reduction in students' maladaptive behaviors of aggression and non-compliance in all three classrooms in the study as noted in the data collected from classroom observations. A social validity survey completed by the teacher participants confirmed the social acceptability of the training program and methods. [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.]