At the LEAR facility, CERN, antiprotonic Lα transitions in light elements have been investigated with a focussing crystal spectrometer. The high resolution of the experiment allowed for the first time to resolve in \({\bar p}\)H the 23P0 state from the close lying states 23P2, 21P1, and 23P1. In \({\bar p}\)D the corresponding transitions were found to be more than an order of magnitude broader. To a large extent the results for \({\bar p}\)H support the meson exchange model.

Some time after its formation an exotic atom may be considered a hydrogen-like system consisting of a nucleus and an exotic particle in a bound state. In this situation it is an ideal tool to study cascade properties, while for the innermost orbits it can be used to probe the interaction with the nucleus. From an extended series of experiments using high resolution X-ray spectroscopy for both aspects typical examples are reported and preliminary results are given: 1. To determine the complex scattering length in \({\bar p}\)H the \({\text{3D}} \to 2{\text{P}} \) hyperfine transitions have been measured. 2. To determine the pion mass the 5 \(\to \) 4 transitions in \(\pi ^{14} \)N have been studied. In all cases a major contribution to the uncertainty originates from the calibration. Therefore a new method is proposed that will establish a universal set of high precision calibration lines for pionic, muonic and electronic systems.