  Differences in the photoproduction of mesons on the free proton and on nuclei
are expected to reveal changes in the properties of hadrons. Double pion
photoproduction from nuclei has been used to investigate the in medium
modification of meson-meson interactions with the TAPS photon spectrometer at
MAMI-B. Recent developments are reported.
  Hadron-hadron interactions and meson production and decays involving light
nuclei are also studied in hadron induced experiments. In particular, the
physics program of the new WASA-at-COSY facility includes double pion
production from light nuclei. Here, the efforts focus on the ABC effect, the
puzzling low-mass enhancement in the pi-pi invariant mass spectrum, first
observed in inclusive measurements of two-pion production in nuclear
collisions.
