We introduce “epistemic identities” as a concept helpful for understanding the dynamics of interdisciplinary science practice. After acknowledging the ambiguity of “identity” and examining divergent meanings, we argue that analysis of identity is necessary in order to account for social and personal dimensions of practice non-reductively, and to better understand values and implicit hierarchies in science practice, as these, in turn, influence cognitive practices. With reference to ethnographic data from two integrative systems biology laboratories, we analyze specifically three theories of identity in terms of their affordances for understanding intergroup characterizations in interdisciplinary problem spaces.