  Mixed-morphology SNRs are characterized by a shell-like radio emission, a
centrally peaked X-ray morphology, and by interaction with molecular clouds.
Many models have been proposed to explain these peculiar remnants, but their
physical origin is still unclear. The recent discovery of over-ionized (i. e.
recombining) ejecta in 3 mixed-morphology SNRs has dramatically challenged all
the previous models and opened up new, unexpected scenarios. I review the main
properties of these remnants and their peculiar X-ray spectral properties. I
also discuss the hydrodynamic model developed to explain the presence of
over-ionized ejecta in W49B and present a list of open issues that still need
to be clarified.
