In many studied materials and mixed compounds, the magnetic response and in particular the energy spectrum of magnons will be strongly influenced by different kinds of disorder. Thin films might consist of islands or alloys might form solid solutions. Hence, we need to take into account disorder as well, in order to describe magnons theoretically. We will present briefly the underlying theory of our two complementary approaches based on a Heisenberg model. The included effective interaction between magnetic moments entering the Heisenberg model can be obtained from first-principles using a self-consistent Green function method within the density functional theory. We demonstrate the application of both methods on experimentally relevant material systems: 3ML Co/Cu(001), 1ML Fe/Pd(001),
and Fe-Co alloys. Taking into account the disorder improves a lot the agreement between the spin-polarized electron energy loss spectroscopy measurements and the theoretical results.