abstract: We consider a periodic Schr\"{o}dinger operator and the composite Wannier functions corresponding to a relevant family of its Bloch bands, separated by a gap from the rest of the spectrum. We study the associated localization functional introduced by Marzari-Vanderbilt and we prove some results about the existence and exponential localization of its minimizers, in dimension $d \leq 3$. The proof exploits ideas and methods from the theory of harmonic maps between Riemannian manifolds.