Basic theory tells us that invasions will eventually reduce biodiversity as similar species are brought into contact. Similar species may compete for shared resources, but other processes can mimic the population effects of competition such as apparent competition through shared enemies. I review evidence for the role of behavior in several ongoing invasions including displacements among Pacific island geckos. These studies illustrate the importance of an experimental approach to understanding invasion dynamics, but they also offer a guide for developing a general predictive framework for invasions. A common theme is that understanding behavioral interactions among individuals is critical for understanding how invaders affect residents.  Experiments have revealed unanticipated behavioral mechanisms of displacement, including some that arise from overlapping species and mate recognition cues. Reproductive interference encompasses several kinds of behavioral interactions that result from “crossed signals” between species. Several types of reproductive interference can lead to faster rates of displacement than ecological interactions such as resource competition. Evidence of reproductive interference, or evidence to rule it out, may be hard to come by without carefully controlled experiments.