  Fifty five planetary nebulae containing micro-structures of low-ionization
(LIS) are analyzed in this review in terms of LIS morphology, kinematics, and
physical and excitation properties. We attempt to address the issue of their
origin through the comparison of LIS properties with the main shells of the
nebulae, as well as the contrast with the theoretical model predictions. We
finally conclude that, while LIS morphology and kinematics can be reasonably
accounted for by the available theoretical models, they cannot explain the LIS
physical/excitation properties, unless we agree that evolved PNe are expected
to show neither shock-excited LIS nor significant density contrasts relatively
to their environments. Some evidence for the latter ideas is actually presented
in this paper.
