Every year consumers voluntarily give away billions of dollars to service workers in the form of tips. The voluntary nature of tipping raises interesting questions about why people tip and what factors influence their tipping decisions. These questions are addressed in a review of theoretical work on tipping and in a meta-analysis of 22 published and 14 unpublished studies examining the predictors of tip size in restaurant settings. Our findings suggest that tipping is predominantly affected by social expectations, server attractiveness, server friendliness and customer mood. Service quality and cost considerations appear to have only weak effects on tipping. The theoretical, methodological, and practical implications of these findings are discussed along with directions for future research.