This mixed-method study examined special educators’ and related services professionals’ overall knowledge and implementation of behavior intervention plans (BIPs), as well as factors that impact the fidelity of BIPs for students with autism spectrum disorders and significant cognitive disabilities in a specialized school. The results of the study suggest inconsistencies between participants’ competency and their actual implementation of BIPs in practice. The findings have consistencies and inconsistencies with existing literature. Additionally, the researcher identified the participants’ perceptions of why behavior plans are not implemented with fidelity and perceptions of how behavior plans are likely to be implemented with fidelity in applied settings. The researcher identified subthemes for barriers, including inadequate preservice coursework and staff development, inadequate resources, insufficient quality of behavior plans, lack of communication among professionals, inconsistent implementation of BIPs, settings where the behavior occurs, and staff comfort level during a behavior. The results of the study indicate that the following subthemes positively influence special educators’ and related services professionals’ implementation of behavior plans: rapport between students and staff, antecedent interventions as a natural part of the day, and collaboration among team members. Lastly, the findings suggest that years since degree earned, level of education, job title, and the age of students on the caseload do not have a significant impact on special educators’ and related services professionals’ ability to implement behavior plans in practice.