Associations between attachment style, hook up behavior, and gender were tested in two studies of undergraduate students using online survey and diary methods. Study 1 (N = 855) consisted of a cross-sectional investigation of attachment style, sexual history, and gender. Study 2 (N = 229) consisted of a weekly diary study completed over the course of an academic semester (12 weeks) to explore the relationship between hooking up and changes in weekly feelings of attachment security, avoidance, and anxiety, using the State Adult Attachment Measure (Gillath, Hart, Noftle, & Stockdale, 2009). Participants with sexual histories that included only hook up partners and no romantic/committed sexual partners rated themselves highest on both attachment avoidance and anxiety. In the diary study, weekly feelings of security and avoidance increased on weeks when a hook up occurred, and there was an interaction between hook up and week on changes in weekly feelings of anxiety. Limitations and future directions for this line of research are discussed.