Based on the physics underlying turbulence anisotropy in the equilibrium and nonequilibrium limits, a new physically-based, fully-realizable, nonequilibrium k -ε RANS model has been developed. The model is based on an effective strain rate tensor that accounts for the strain history to which the turbulence has been subjected. This new model is applied to four distinctly different test cases for which the nonequilibrium history integral can be evaluated analytically. Results obtained from this new closure model show dramatically improved agreement with experimental and computational data when compared with the standard k -ε (SKE) model, without the need to vary any model parameters. The introduction of a nonequilibrium effective strain rate allows this new model to be applied within a similar framework as currently used for two-equation eddy viscosity models, thereby permitting relatively simple implementation in existing CFD codes.