The viability of using brief observations of behavior (thin slicing) to identify infidelity in romantic relationships was examined. Two studies supported the hypothesis that observers can accuratelsy identify people who are cheating on their romantic dating partner based on thin slices of observed behavior. In Study 1, raters were able to accurately identify people who were cheating on their romantic dating partner after viewing a short 3- to 4-min video of the couple interacting. Study 2 replicated this finding and identified possible variables that may mediate the relation between coder's ratings and participants' actual reported infidelity. Commitment and trustworthiness were found to be mediators of this relation. These results are discussed in terms of application and future research.