  We discuss the cosmology of models in which the standard model Yukawa
couplings depend on scalar field(s), often referred to as flavons. We find that
thermal corrections of the flavon potential tend to decrease the Yukawa
couplings, providing an important input to model-building. Working in the
specific framework of Froggatt-Nielsen models, we compute the abundance of
flavons in the early universe generated both via freeze-in and from coherent
oscillations induced by thermal corrections to their potential, and discuss
constraints on flavon models from cosmology. We find that cosmology places
important constraints on theories containing flavons even for regions of
parameter space inaccessible to collider searches.
