In this contribution we first present a short introduction into the lattice structure, the phase diagram and the electronic structure of high-T_c superconductors. Then we explain the principles of angle-resolved photoemission spectroscopy (ARPES) and the influence of the dressing of the charge carriers, which is normally described by the complex self-energy function, on the spectral function in the normal and the superconducting state. Finally we review our recent ARPES results on high-T_c superconductors at various k-points in the Brillouin zone near the Fermi surface. Information on the dressing of the charge carriers, i.e., on the effective mass and the scattering length, is obtained as a function of doping concentration, temperature, momentum and energy. The strong renormalization of the bandstructure due to the dressing can be explained in terms of a coupling to a continuum of spin fluctuations and in the superconducting state by an additional coupling to a triplet exciton excitation. Possibly, this dressing is related to the glue for the pair formation in cuprate superconductors.