Many-particle electron-phonon interaction effects in two-dimensional electron gases are investigated within a Born–Markov approach. We calculate the electron-phonon interaction on a microscopic level to describe relaxation processes of quantum confined electrons on ultrafast time scales. Typical examples, where two-dimensional electron gases play a role, are surfaces and two-dimensional nanostructures such as graphene and quantum wells. In graphene, we find nonequilibrium phonon generation and ultrafast cooling processes after optical excitation. Electron relaxation dynamics at the silicon (001) 2×1 surface exhibits two time scales, corresponding to intrasurface and inside bulk-scattering processes. For GaAs quantum wells, we present broad emission spectra in the terahertz range assisted by LO-phonons of the barrier material.