Since ancient times, humans have lived and even prospered in the arid regions of South Arabia, where the pre-eminent factor limiting life is water. How did people in the past interact with a natural environment that seems so uninviting, indeed hostile towards humans? What kinds of strategies did they use in order to deal with the scarce water resources? Did their practices change over time, or did the natural conditions restrict the options severely? There arc numerous sites in South Arabia that can serve as examples for skillful hydraulic engineering and water management, both from ancient and medieval Islamic times. The following considerations are based on two case studies: first, the ancient oasis of Marib that flourished during the first millennium BCE and was abandoned around 600 CE, just before the coming of Islam; and, second, the city of Zabid, founded in the ninth century CE, with its agricultural hinterland that has been farmed continuously since then. These two sites where the author has conducted field research since the 1980s also allow us to address the question of continuity and change from pre-Islamic to Islamic times. In order to appreciate what people in the past accomplished and still do, it is necessary to start with a brief review of what aridity means in terms of the water resources.