The principles of planing on the water surface are introduced beginning with the analogy to a wing or hydrofoil. The equations of motion and boundary conditions are developed, and the gravity effect that is important for lower speeds where buoyancy also plays a part is described. Development of the slender wing analogy and the effect of viscous flow on planing surface forces together with the effect of gravity and buoyancy forces are also described. The effect of planing surface shape is discussed, including curvature, deadrise, and varying breadth.