A pairwise trading process is formulated subject to conditions of non-negativity of traders' holdings, quid pro quo, and a limited number of trading opportunities. The following points are made: (i) there is a centralized procedure that achieves the equilibrium allocation for an arbitrary economy; (ii) it is not in general possible to find a decentralized procedure that achieves the equilibrium allocation for an arbitrary economy; and (iii) in a monetary economy there is a decentralized procedure that achieves the equilibrium allocation. The usefulness of money is that it allows decentralization of the trading process.