In its most general form the Lucas Washburn equation describes the penetration length (formula_1) of a liquid into a capillary pore or tube with time formula_2 as formula_3, where formula_4 is a simplified diffusion coefficient. This relationship, which holds true for a variety of situations, captures the essence of Lucas and Washburn's equation and shows that capillary penetration and fluid transport through porous structures exhibit diffusive behaviour akin to that which occurs in numerous physical and chemical systems. The diffusion coefficient formula_4 is governed by the geometry of the capillary as well as the properties of the penetrating fluid.