This Demonstration calculates the thickness of a boundary layer for flow between parallel plates as a function of distance down the plates. You can vary the distance between the plates, the fluid velocity, and the kinematic viscosity. The boundary layer (the shaded light blue region) represents the region where viscous forces must be taken into account due to the no-slip condition. Outside of the boundary layer (the white region), viscous forces are negligible. Once the two boundary layers meet midway between the plates, the fluid flow is fully developed.