The presence of water is one of the most critical factors contributing to the instability of hillslopes. A common solution to stabilize hillslopes is installation of horizontal drains to decrease the elevation of the water table surface. Lowering the water table dries a large portion of the hillslope which increases the shear strength of the soil, thereby decreasing the probability of slope failure. The purpose of this manual is to provide a single comprehensive reference for geotechnical engineers and hydrogeologists on designing horizontal drainage systems to improve slope stability. Guidelines are provided for translational and rotational failure and consider fractured systems. Basics of hydrogeologic and geotechnical terminology, site characterization and conceptualization, groundwater modeling techniques and template projects help to guide the user with respect to identifying important parameters to drainage design. An iterative approach is presented for determining the minimum drain construction to lower water levels enough to keep the factor of safety (FOS) greater than 1.2.