These essays describe the diffusion of hydraulic institutions and techniques from the Islamic world into medieval Spain and, then, on to the Canaries and to America. Professor Glick's concern is both with technical aspects of water use, such as mills and flow measurement, and with the social basis of the technology; and it is this latter aspect that he would see as the key to success; Given the critical importance of water for agriculture, and also often competing urban and industrial uses, means to ensure equitable distribution and the management of disputes were essential. The mechanisms used to achieve this are analysed in the light of the objectives of local irrigation communities.