In arid and semi-arid regions of the world, rainfall is supplemented by irrigation, and drainage is needed to prevent irrigation-induced waterlogging and soil salinization. Drainage systems are man-made systems that are only implemented when natural drainage is insufficient for a satisfactory form of agriculture. This chapter provides an overview of agricultural drainage systems, explains in detail the need for drainage and its benefits, and explores in some depth the challenges of making drainage work, including the construction of drainage infrastructure and ways of dealing with drainage effluent. The chapter looks ahead to future trends in this area of research and provides detailed guidance on further sources of information.