  The present status of the in-medium invariant $\pi-\pi$ mass distribution in
the sigma-meson channel around the threshold is reviewed from the theory side
and contrasted with recent experiments on $(\pi,2\pi)$ knock-out reactions. A
preliminary investigation indicates that the strongly target-mass dependent
invariant mass enhancement of the two pions can be explained theoretically. A
more refined reaction theory is needed to confirm this result. In the
theoretical description, based on the linear sigma model, emphasis is put on
constraints from chiral symmetry.
