Low energy QCD with light u, d quarks has reached by now the precision of a quantitative science. In this sector there is a rich, high‐precision experimental data set available, and chiral perturbation theory with the pion as a good approximation of the Nambu‐ Goldstone boson works pretty well, and is also in a good agreement with Lattice QCD.

The situation in the strangeness sector, however, is quite opposite. The basic low energy KN interaction has a difficulty in the theoretical treatment additionally due to the strong coupling between KN and πΣ channels.

Two modern precise measurements of the kaonic hydrogen energy shift and width from KpX at KEK and DEAR at LNF‐INFN set empirical constraints on calculation of the KN interaction. A chiral SU(3) unitary approach with coupled‐channel has been applied and reproduces both data, however, it suffers from uncertainties of the data which meanwhile became not satisfactory, showing a minor discrepancy between the two results (Fig. 1).

The SIDDHARTA experiment has been launched in 2008 and this situation is expected to be solved. SIDDHARTA aims at improving the precision of the kaonic hydrogen data significantly, and also to measure a kaonic deuterium for the first time to examine the isospin dependence of the K‐N interaction.

The KEK‐PS E570 experiment determined the 2p energy shift of kaonic 4He. The existing old data were inconsistent to other kaonic atom measurements, which had been a long standing puzzle. The newly arrived accurate data agrees with the other kaonic atom data, and preliminary data from SIDDHARTA confirmed this result (see Fig. 2).