Pionic Hydrogen

K X-rays from pionic hydrogen and deuterium as well as muonic hydrogen have been measured using a highresolution crystal spectrometer at the _E5 beam line of the Paul Scherrer Institut. From the ground-state level shifts and broadenings of the hydrogen isotopes, caused by the strong interaction, low-energy parameters of QCD become accessible as are the pion-nucleon scattering lengths and the threshold production strength of pions in nucleon-nucleon collisions. Muonic hydrogen allows important insights in the de-excitation cascade of such exotic atoms, the understanding of which is essential for a precision determination of the above-mentioned quantities. First experimental results are discussed in the context of recent results from theoretical e_orts within the approach of chiral pertubation theory and atomic cascade calculations.