  Understanding hadronic interactions is crucial for investigating the
properties of unstable hadrons, since measuring physical quantities for
unstable hadrons including the resonance mass and decay width requires
simultaneous calculations of final scattering states. Recent studies of
hadronic scatterings and decays are reviewed from this point of view.
  The nuceon-nucleon and multi-nucleon interactions are very important to
understand the formation of nucleus from the first principle of QCD. These
interactions have been studied mainly by two methods, due originally to
L\"uscher and to HALQCD. The results obtained from the two methods are compared
in three channels, $I=2$ two-pion, H-dibaryon, and two-nucleon channels. So far
the results from the two methods for the two-nucleon channels are different
even at the level of the presence or absence of bound states. We then discuss
possible uncertainties in each method.
  Recent results on the binding energy for helium nuclei are also reviewed.
