Tehran is situated on an alluvial plain immediately south of the Alborz Mountains which form an east-west chain stretching across northern Iran. The alluvium is composed of an older, folded group, of probable Pliocene age, which is overlain by flat-lying Quaternary gravels, sands, silts and boulder beds. Recharge from surface streams takes place in the areas underlain by the flat-lying alluvium. The pattern of ground-water flow is related to the sub-surface structure of the alluvium. Chemical data are presented which indicate that extensive surface recharge takes place below the city. The ground-water balance is discussed and it is concluded that, at present, there is a balance between inflow and abstraction.