  Using the high-quality beam of storage rings in combination with a pellet
target and the hermetic WASA detector covering practically the full solid angle
the two-pion production in nucleon-nucleon collisions has been systematically
studied by exclusive and kinematically complete measurements -- first at
CELSIUS and subsequently at COSY. These measurements resulted in a detailed
understanding of the two-pion production mechanism by $t$-channel meson
exchange. The investigation of the ABC effect in double-pionic fusion reactions
lead the trace to the observation of a narrow dibaryon resonance with $I(J^P) =
0(3^+)$ about 80 MeV below the nominal mass of the conventional $\Delta\Delta$
system. New neutron-proton scattering data, taken with polarized beam at COSY,
produce a pole in the coupled $^3D_3$ - $^3G_3$ partial waves at
($2380\pm10~-~i~40\pm5$) MeV establishing thus the first observation of a
genuine $s$-channel dibaryon resonance.
