This lecture covers the principles of groundwater flow and flow nets. It begins with three-dimensional flow, explaining how specific discharge is treated as a vector quantity and described using Darcy’s law, especially in homogeneous and isotropic formations. The direction of groundwater movement follows the hydraulic gradient, with flow lines perpendicular to contour lines on water table or piezometric surface maps and determining movement direction often requires data from multiple wells. Flow nets, constructed by drawing flow lines and equipotential lines perpendicular to each other, provide a graphical method for analyzing groundwater flow, especially useful in civil engineering for problems involving hydraulic structures. The lecture also explains how these nets are used to visualize and solve groundwater flow problems by creating a curvilinear grid that models the flow paths and hydraulic head distribution.