Although researchers have examined the predictors and outcomes of the behavioral aspect of "hooking up," typically defined as casual, commitment-free sexual encounters, research has not yet examined the extent to which young people endorse the culture associated with hooking up. Based on the argument that there is a set of understood rules and assumptions associated with hooking up, this article describes the development of an instrument to measure college students' endorsement of the hookup culture. Results from two studies found that five factors represent endorsement of the hookup culture: (a) a belief that hooking up is harmless and best without emotional commitment, (b) a belief that hooking up is fun, (c) a belief that hooking up will enhance one's status in one's peer group, (d) a belief that hooking up allows one to assert control over one's sexuality, and (e) a belief that hooking up is a reflection of one's sexual freedom. The index also had acceptable internal reliability, and performed well on the tests of construct validity. Taken together, the results suggest that the Endorsement of the Hookup Culture Index is suitable for use by researchers interested in the social phenomenon of hooking up.