Despite the availability of low-cost exchanges, over-the-counter (OTC) trading is pervasive for most assets. We explain the prevalence of OTC trading using a model of adverse selection, in which informed and uninformed investors choose to trade over-the-counter or on an exchange. OTC dealers' ability to price discriminate allows them to imperfectly cream-skim the uninformed investors from the exchange. Assets with lower adverse selection risk are predicted to have a higher fraction of trades over-the-counter, as observed in practice. The presence of an OTC market increases aggregate trade volume and reduces average bid-ask spread; nonetheless, welfare declines when adverse selection risk is low.Surprisingly, for assets that are mostly traded over-the-counter, closing the OTC market improves welfare, providing support for recent regulatory efforts to end OTC trading in those assets. JEL-Classification: D47, G14, G18, G23