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The two-pion production in nucleon-nucleon collisions has been studied byexclusive and kinematically complete experiments from threshold up to $T_p$ =1.36 GeV at CELSIUS-WASA. At near-threshold energies the total and differentialdistributions for the $\pi^+\pi^-$ and $\pi^0\pi^0$ channels are dominated byRoper excitation and its decay into $N\sigma$ and $\Delta\pi$ channels. At beamenergies $T_p >$ 1.1 GeV the $\Delta\Delta$ excitation governs the two-pionproduction process. In the $\pi^+\pi^+$ channel evidence is found for theexcitation of a higher-lying I=3/2 resonance, favorably the $\Delta(1600)$. Theisovector fusion processes leading to the deuteron and to quasi-stable $^2$He,respectively, %with the production of an isovector pion-pair exhibit no or onlya modest ABC-effect, {\it i.e.} low-mass enhancement in the $\pi\pi$-invariantmass spectrum, and can be described by conventional $t$-channel $\Delta\Delta$excitation. On the other hand, the isoscalar fusion process to the deuteron%with the production of an isoscalar pion-pair exhibits a dramatic ABC-effectcorrelated with a narrow resonance-like energy dependence in the total crosssection with a width of only 50 MeV and situated at a mass 90 MeV below the$\Delta\Delta$ mass.

Systematic Study of Two-Pion Production in NN Collisions -- from Single-Baryon to Di-Baryon Excitations