Undergraduate programs across the nation are increasingly offering interdisciplinary study programs as markers of their commitment to educate individuals for the demands of contemporary life. Yet, as students engage in interdisciplinary learning projects, an unaddressed question looms large: how to adequately assess student interdisciplinary work. How can faculty, trained to be disciplinary experts, properly determine what constitutes quality work when familiar disciplinary standards do not suffice? Adequately assessing student learning in higher education remains more a matter of collective hope than of convergent and well-tested practice. The issue is marred by controversies over the purposes, methods, and most importantly, the content of proposed assessments. In this article, the author proposes a definition of interdisciplinary understanding and a framework to inform the assessment of student interdisciplinary work. The arguments presented stem from an empirical study conducted at the Harvard Interdisciplinary Studies Project. It examines interdisciplinary research and educational practices in well-recognized research centers and educational programs like the Media Lab at MIT, the Center for Bioethics at the University of Pennsylvania, and the Human Biology Program at Stanford University. Assessment of student interdisciplinary understanding was a central focus of this analysis of 50 faculty interview transcripts and more than 50 pieces of student work. Interdisciplinarity is an elusive concept. Stated definitions in the literature are varied, as are the enacted definitions that tacitly guide real teaching practices. The term is employed to describe a broad array of endeavors ranging from a biochemistry student learning about gene regulation, to a faculty member using the visual arts to introduce a mathematical concept, to a student's post-structuralist critique of the nature of disciplinary authority. This semantic elusiveness is exacerbated by the fact that current scholarly debates about interdisciplinarity involve social, political, cognitive, and epistemological dimensions. In interdisciplinary work, many possible integrations are viable. For example, autism can be explored at the crossroads of psychology and sociology by examining the unique forms of social discrimination associated with autistic children. Or it could be investigated at the crossroads of neurology and medical ethics--if one were to consider experimenting with novel medical procedures. The merit of an interdisciplinary integration should be assessed against the specific goal of each interdisciplinary enterprise. A rigorous account of the nature of interdisciplinary understanding--the epistemological foundations on which it stands and the cognitive challenges it presents--provides a blueprint for examining student interdisciplinary work, to find evidence of accomplishment, and to identify ways to support improved understanding.