Equations describing fully developed turbulent flow in an annulus are derived. They are based upon Reichardt’s expression for the eddy diffusivity of momentum, and they assume that the velocity profiles starting from the core tube wall and the outer tube wall have the same velocity and eddy diffusivity at the plane of zero shear. The predicted location of the plane of zero shear, mixing length, eddy diffusivity, velocity distribution, and friction factor are compared to available data and are found to give good agreement with the test results. Potential extension of the proposed method to more complex geometries is illustrated by considering the case of flow in an annulus with one artificially roughened surface.