  The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ invariant
mass spectrum - is known from inclusive measurements of two-pion production in
nuclear collisions, where it always showed up, if the participating nucleons
fused to a bound nuclear system in the final state. The first exclusive
measurements on the ABC effect have been carried out very recently at
CELSIUS-WASA for the fusion reactions leading to d, $^3$He and $^4$He nuclei in
the final state. The data analyzed so far for the fusion processes to d and
$^3$He reveal this effect to be a $\sigma$ channel phenomenon associated with
the formation of a strongly attractive $\Delta\Delta$ system. The data for the
strictly isospin-selective double-pionic fusion to $^4$He, where we expect the
largest effect, are currently still analyzed. All inclusive data on this system
are well described by our model, too. This case also constitutes the heaviest
nuclear system, where exclusive measurements of double-pionic fusion can be
carried out with present-day instruments. Surprisingly, the $pp \to
pp\pi^0\pi^0$ reaction in the $\Delta\Delta$ region is observed to also show a
ABC-like low-$\pi\pi$ mass enhancement, a phenomenon, which deserves special
attention.
