Rotation braking is now recognized as fundamental to the understanding of stellar evolution. Photometric monitoring programs have mapped the mass-dependent range of pre-main sequence rotation rates and have shown that star-disk interactions (planet and star formation processes) are key to the
subsequent evolution of stellar rotation. It is known that Sun-like stars spin down from magnetized winds. Yet recent large samples of active stars now show that the stellar-mass dependence of these winds is uncorrelated with the boundary at which stars become fully convective, contrary to theoretical expectations. Helioseismology has revealed the internal solar rotation, invigorating debate over dynamo theories and confirming the strong coupling between the radiative core and convective envelope. However, mechanisms for angular-momentum transport in stellar interiors, the related mixing, and relevant timescales are still poorly understood. Nascent asteroseismic constraints on internal stellar rotation exist, but crucial knowledge, such as the core rotation rate in pre-supernova stars, is lacking. Understanding rotation-induced mixing in stellar interiors and its effect on stellar and chemical evolution demands that these uncertainties be resolved. It is known that magnetic fields in
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