The authors explored the psychology of romantically attracting someone who is already in a relationship--what can be called the process of human mate poaching. In Study 1 (N = 236), they found that attempts at poaching were relatively common and were linked with distinctive personality dispositions. Study 2 (N = 220) documented that the perceived costs and benefits of poaching differed somewhat for men and women and depended on whether short-term or long-term poaching outcomes were targeted. Study 3 (N = 453) found support for 5 evolution-based hypotheses about the perceived effectiveness of poaching tactics. Study 4 (N = 333) found that poaching effectiveness was influenced by the type of relationship being encroached on-marital, dating, long distance, highly committed, just beginning, or about to end. Discussion focuses on the importance of placing mate poaching within the broader context of human sexual strategies.