This chapter analyses the functioning and evolution of the pósitos, the system of grain banks in Spain from the sixteenth to the nineteenth centuries. It examines the economic logic of storage and cereal speculation, distinguishing between short- and long-term operations, and assessing their profitability through econometric models and price data. The study explores the development of pósitos as municipal public institutions, designed both to support farmers in years of poor harvests and to guarantee affordable urban food supplies. Although they expanded significantly during the eighteenth century, their profitability was constrained by state regulation, price ceilings, and restrictions on private trade. Their final decline, accelerated by agricultural crises and wars, transformed the pósitos into primarily charitable rather than economic entities, revealing the limited success of the system.