The low-energy anti-kaon nucleon system is of special interest as a testing ground for chiral SU(3) symmetry in QCD. The existence of the Lambda(1405) resonance just 25 MeV below the K^-p threshold, however, makes chiral perturbation theory inapplicable in this channel.
Non-perturbative coupled-channel techniques based on driving terms of the chiral SU(3) effective Lagrangian have proved useful and successful in dealing with this problem. Taking K^-p scattering data as input we provide an error range for the strong K^-p scattering length which is related to the strong interaction shift and width in kaonic hydrogen. The derived strong interaction shift and width is in agreement with the KEK experiment, but is in clear disagreement with the one deduced from the DEAR experiment.
Our investigations are also of considerable interest in the discussion of possible deeply bound K^- nuclear states.