At the end of a long water conductor system either a surge tank or a forebay is provided and their design details are dealt in this chapter. Surge tank is provided at the end of an under-pressure tunnel. In case of a free flow-tunnel and a channel a forebay is provided. Both these structures supply flow to the turbine for the time by which the flow is accelerated in the water conductor. These structures are located as near to the power house as possible. The features, layout and design principles of various types of surge tanks are described. The forebay is a widened and deepened channel section with an intake for penstocks at its downstream end. Its layout and design features are also briefly described in this chapter.