Engagement in early childhood is defined as a child's level of participation with the environment. Engagement is an important construct in assessment and intervention of social and early learning competence given its link to school achievement. Few tools exist to assess engagement of young children in early education, and there is a need for a systematic direct observation tool that can be applied universally (e.g., with all young children) in these settings. This article describes preliminary evidence of validity and reliability for the Behavioral Observation of Students in Schools--Early Education (BOSS-EE). Specifically, the article describes results from a survey of experts and practitioners in which feedback was solicited on target behaviors and operational definitions, presents reliability data (i.e., interobserver and test-retest), examines correlations with a criterion measure, and describes variability across settings, sites, and methods (i.e., video vs. in vivo). Next steps in measurement development are discussed with attention to the challenges of producing a tool that can be used in a range of early education settings with diverse groups of young children.