Tipping is a world-wide custom involving billions of dollars. The voluntary nature of tipping raises questions about why people tip. From a rational-choice perspective, tipping makes sense only if desired outcomes are contingent on how much is tipped. Three possibilities are that people tip in order to buy social approval, equitable relationships and/or future service. Hypotheses derived from these potential explanations for tipping were tested in a study in which restaurant customers were interviewed (as they left the restaurant) about their dining experience and tipping behavior. Tipping was related to bill size, patronage frequency, service ratings and the interaction of bill size with patronage frequency. Tipping was not related to group size, number of courses, alcohol consumption, food ratings or the interaction of patronage frequency with service ratings. These results are consistent with the use of tips to buy social approval and equitable relationships but not with the use of tips to buy future service.