Schoolwide positive behavioral interventions and supports (SWPBIS) are becoming increasingly popular with schools across the country to help create safer learning environments for students. An important aspect of SWPBIS is the ongoing monitoring and evaluation of implementation fidelity. Although a few measures have been created to assess the degree to which schools are implementing the key aspects of SWPBIS, there remains a need for a tool to categorize a school's overall "phase" of implementation and document the schools' progression toward sustainability of SWPBIS. The present study examines the reliability and validity of the "Implementation Phases Inventory" (IPI), a new measure of SWPBIS implementation fidelity. The findings indicate that the IPI is an internally consistent measure with adequate test-retest reliability, interrater reliability, and concurrent validity. These findings provide preliminary evidence that supports the use of the IPI to assess phase of SWPBIS implementation. (Contains 6 tables and 2 figures.)