This study examines the psychometric properties of an observer measure of developmentally appropriate practices (DAP) for early elementary school classrooms, known as A Developmentally Appropriate Practices Template (ADAPT). Using a sample of 854-1511 first through third grade classrooms in 207-295 schools across 3 years, the study evaluates the reliability and content validity of the ADAPT as a measure of DAP; examines the ADAPT ratings descriptively, including the extent to which classrooms within the same schools are similar; evaluates the factor structure of the ADAPT; tests the invariance of that structure across treatment conditions; and assesses the convergent validity of the measure. The ADAPT is found to have a reliable, stable, and consistent factor structure consisting of three distinct but correlated factors: integrated curriculum, a social emotional emphasis, and child-centered approaches. The ADAPT was also found to be moderately related to another observational measure of the broader classroom environment. There was a large degree of similarity in DAP among classrooms within schools, affirming the need for using a multilevel framework to assess these practices.