  Stars in disks of spiral galaxies are usually assumed to remain roughly at
their birth radii. This assumption is built into decades of modelling of the
evolution of stellar populations in our own Galaxy and in external systems. We
present results from self-consistent high-resolution $N$-body + Smooth Particle
Hydrodynamics simulations of disk formation, in which stars migrate across
significant galactocentric distances due to resonant scattering with transient
spiral arms, while preserving their circular orbits. We investigate the
implications of such migrations for observed stellar populations. Radial
migration provides an explanation for the observed flatness and spread in the
age-metallicity relation and the relative lack of metal poor stars in the solar
neighborhood. The presence of radial migration also prompts rethinking of
interpretations of extra-galactic stellar population data, especially for
determinations of star formation histories.
