In addition to developing deep knowledge of a single discipline, engineers must also be able to collaborate across disciplinary boundaries and develop interdisciplinary expertise to successfully address the complex challenges of the contemporary workplace. While numerous descriptions of interdisciplinary courses and projects appear in the literature, educators still lack rigorous research about learning barriers, outcomes, and concrete interventions to support this interdisciplinary development. This paper addresses that gap by pairing a review of the literature with a case study of students in a sustainable engineering program to identify the key challenges to success in interdisciplinary contexts. The findings suggest that students (1) lack the ability to connect interdisciplinary subjects to their own more narrowly defined fields of expertise, and (2) fail to identify and value the contributions of multiple fields to complex problems. This paper concludes with possible teaching interventions to address these barriers. (Contains 6 tables and 2 figures.)