Research on factors that may promote engagement is hampered by the absence of a measure of classroom-level engagement. Literature has suggested that engagement may have 3 dimensions--affective, behavioral, and cognitive. No existing engagement scales measure all 3 dimensions at the classroom level. The Classroom Engagement Inventory (CEI) was developed to fill this gap. In Study 1, exploratory and confirmatory factor analyses were conducted on data from 3,481 students from the 4th to 12th grade. The results suggested a 4-factor model of the CEI. Using these results, in Study 2 several items were revised and data were collected 1 year later from 4th to 12th grade students in the same school district as Study 1. Analyses were conducted on data from 3,560 students after data cleaning. A series of potential models was tested. The final results suggest a 5-factor 24-item CEI: (1) Affective Engagement, (2) Behavioral Engagement--Compliance, (3) Behavioral Engagement--Effortful Class Participation, (4) Cognitive Engagement, and (5) Disengagement. Results advance understanding of the construct of classroom engagement. The CEI fills a significant gap in measurement of engagement. The CEI is classroom level, measures multiple dimensions of engagement, uses self-report, is relatively short, and can be readily administered in classrooms from the 4th to 12th grade.