Stars in disks of spiral galaxies are typically 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. I will 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. I will also present observational evidence which, in conjunction with our models, strongly suggests that radial migration is an important mechanism in real systems. I will then demonstrate the implications of such migrations for observed stellar populations and argue that radial migration is a fundamental effect that must be considered at all levels of stellar population studies. As such, radial migration provides a new explanation for the observed flatness and spread in the age-metallicity relation and a revised solution to the G-dwarf problem. The presence of radial migration also prompts rethinking of interpretations of extra-galactic stellar population data, especially for determinations of star formation histories.