In this talk, I will present a generalized finite element method with optimal local approximation spaces for solving heterogeneous Helmholtz problems with large wavenumbers. The local spaces are built from selected eigenvectors of carefully designed local eigenvalue problem. Besides being a multiscale discretization scheme, the method can be used as a model reduction method at the discrete level. Wavenumber-explicit exponential convergence rates are established for the method at both the continuous and discrete levels. Furthermore, the method is formulated as a two-level Schwarz preconditioner, and a wavenumber explicit convergence rate is proved for the preconditioned GMRES. Numerical results are provided to confirm the theoretical analysis and the efficiency of the proposed methods.