Evolution of Nuclear Shells due to the Tensor Force

The monopole effect of the tensor force is presented, exhibiting how spherical single-particle energies are shifted as protons or neutrons occupy certain orbits. An analytic relation for such shifts is shown, and their general features are explained intuitively. Single-particle levels are shown to change in a systematic and robust way, by using the $\pi +\rho$ meson exchange tensor potential, consistently with the chiral perturbation idea. Several examples are compared with experiments.