Although dry farming is important in Azerbaijan, Kurdistan, and Khorasan, as well as some other districts, a large proportion of Iran’s agriculture has always depended upon irrigation. Approximately half the annual grain crop, and an overwhelming proportion of other crops, are irrigated. With the exception of the Caspian littoral almost the whole of Iran is classified as semi-arid or arid. As is characteristic of lands under this classification, precipitation is generally not only scanty (most of the country has an average of less than 400 mm per year) and poorly distributed through the year, but irregular and unreliable. Irrigation affords not only the necessary soil moisture for productive agriculture, but the regularity and dependability of agricultural production that facilitates settled life and allows the development of large settled populations. Given the availability of refillable and accessible water sources, especially in combination with soil deposits of good quality, irrigation technologies can greatly increase the productivity and carrying capacity of the land. However, the use of these technologies imposes social and economic conditions on the populations that become dependent on them. Irrigation has played an important role in Iranian history and civilization. It involves factors that are significant in the development of settlement pattern and morphology, in the socioeconomic relations of agricultural activities, in the political processes and legal formulations that are based on these relations, and in the associated linguistic, symbolic, and ritual forms of activity that evolve in the cultural elaboration of them.