Positive behavior support (PBS) is an evidence-based framework for improving quality of life and preventing challenging behavior that is often implemented in combination with person-centered practices in the intellectual and developmental disability (IDD) field. Recently, state agencies and organizations supporting people with IDD have adopted a system-wide approach to PBS, much like multi-tiered school-wide PBS for youth in educational settings (e.g., SW-PBIS). Fidelity tools for guiding team implementation and action planning are essential throughout every phase of implementation in any innovation. Although such tools exist for SW-PBIS, no widely used measure currently exists for assessing the implementation of organization-wide PBS in the adult IDD field. This article describes the initial development of a fidelity of implementation tool used to evaluate organization-wide implementation of PBS and person-centered practices in community settings. A Midwestern state Department of Human Services and a university Center on Developmental Disabilities (UCEDD) in the United States partnered to complete this project. The authors discuss the exploratory and descriptive evaluation process involved in the initial development of the tool and share data for 10 provider organizations over a 6-year period of implementation. A descriptive case study using mixed measures is used to provide more details about the implementation of PBS in one agency to show the types of changes captured by the fidelity tool.