  Planetary nebulae (PNe) are known to possess a variety of small-scale
structures that are usually in a lower ionization state than the main body of
the nebulae. The morphological and kinematic properties of these low-ionization
structures (LISs) vary from type to type in the sense that LISs can appear in
the form of pairs of knots, filaments, jets, and isolated features moving with
velocities that either do not differ substantially from that of the ambient
nebula, or instead move supersonically through the environment. The
high-velocity jets and pairs of knots, also known as FLIERs, are likely to be
shock-excited. So far, most of the FLIERs analyzed with ground-based small and
medium telescopes, as well as with the HST, do not show the expected
shock-excited features --either the bow-shock geometry or the shock excited
emission lines. In this talk we discuss the crucial problem of the excitation
mechanisms of FLIERs --through the comparison of jets and knots of NGC 7009 and
K 4-47-- and what might be the contribution of large telescopes.
