We investigate the nonlinear dynamics of turbulent shear flows, with and without rotation, in the context of a simple but physically motivated closure of the equation governing the evolution of the Reynolds stress tensor. We show that the model naturally accounts for some familiar phenomena in parallel shear flows, such as the subcritical transition to turbulence at a finite Reynolds number and the occurrence of a universal velocity profile close to a wall at large Reynolds number. For rotating shear flows we find that, depending on the Rayleigh discriminant of the system, the model predicts either linear instability or nonlinear instability or complete stability as the Reynolds number is increased to large values. We investigate the properties of Couette–Taylor flows for varying inner and outer cylinder rotation rates and identify the region of linear instability (similar to Taylor's), as well as regions of finite-amplitude instability qualitatively compatible with recent experiments. We also discuss quantitative predictions of the model in comparison with a range of experimental torque measurements. Finally, we consider the relevance of this work to the question of the hydrodynamic stability of astrophysical accretion disks.