Background: There is a need for engineering-specific theories and constructs to advance our understanding of student development. Student integration, widely used in educational research, is a suitable construct; however, it has also received some criticism. These critiques can be addressed by rethinking student integration and its use in the context of engineering education. Purpose/Hypothesis: This article re-conceptualizes student integration and describes the development of the Engineering Student Integration Instrument (ESII), which measures integration, as well as provides evidence of its validity. Design/Method: Preliminary items for the ESII were developed using open-ended survey responses from undergraduate engineering students about the impact of co-curricular support. These items were reviewed by multiple researchers to assess question phrasing, formatting, and face validity. The resulting instrument was administered electronically to students in the College of Engineering at a large East Coast public university. Data from 586 students were used to develop evidence of validity by conducting an exploratory factor analysis on half of the data and a confirmatory factor analysis on the other half. Results: Evidence of validity indicates the appropriateness of the ESII for measuring integration and operationalizes integration constructs from the model of co-curricular support (MCCS) for undergraduate engineering students, including academic, social, professional, and university integration. Conclusions: Grounded in the MCCS, the ESII begins to address several critiques of student integration as originally defined by Tinto's model of institutional departure, those related particularly to the measurement of the integration constructs.