The paper develops an explanation for the emergence of media of exchange through the unconcerted market behavior of individuals. Individuals are assumed to accomplish their ultimate exchanges through trading sequences which minimize the expected time spent searching for complementary trading partners. If individual perceptions of the trading environment are appropriately restricted, then the equilibrium pattern of trade will be some mixture of direct barter and use of a common good as medium of exchange. Although full monetization is always a locally stable exchange pattern, the economy may remain in universal direct barter or partially monetized states.