Abstract: England from 1200 to 1600 was a society caught in apparent technological stasis, typical of the pre-industrial world. Many believe this pre-industrial stagnation was the result of political and cultural constraints, such as those on the operation of markets. Indeed medieval English law outlawed many arbitrage activities in markets. The paper shows using information of grain yields and prices at 227 different locations that the most important of all markets, that for grain, was nevertheless both extensive and relatively efficient as early as 1209. Whatever the rhetoric of medieval law and social thought the real effect of constraints on the operation the grain market seems to have been minimal. England had an elaborate market economy at least 500 years before it had sustained economic growth.