Pion production and absorption in piN and 3N systems

We study pion-induced two-pion production on the nucleon in the framework of a relativistic meson-exchange model constrained by Chiral Perturbation Theory in the low-energy limit. Unitarisation effects are included for the resonance pole diagrams. This leads to a significant improvement over a simple tree-level model in the description of the angular distributions for the $\pi^{+}$p $\rightarrow$ $\pi^{+}\pi^{+}\eta$ reaction. In the reaction channels $\pi^{-}$ $\rightarrow$ $\pi^{+}\pi^{-}\eta$ and $\pi^{-}$p $\rightarrow$ $\pi^{0}\pi^{0}\eta$ we find a contribution of the two-pion decay of the Roper resonance. This contribution does however not manifest itself in a qualitative change in the description of the differential distributions. In the second part of this thesis we use the pion as a probe to study the short range part of the three-nucleon wave function in $\pi^{+}$ absorption on $^{3}$He. We employ a parametrisation of the three-nucleon wave function based on modern NN models. The use of this parametrisation leads to a significant improvement over previous calculations in the description of cross section and polarisation data. However, the polarisation data hint at the need of other mechanisms beyond a two-nucleon absorption mechanism for the description of positive pion absorption on $^{3}$He even in quasifree kinematics