In this contribution we discuss new results of a program aimed at studying the physical properties, origin and evolution of ``low-ionization, small-scale structures in PNe''. Within this project we classify and discuss the properties of a sample of more than 50 PNe, collected from the literature, in the light of their possible formation mechanisms. In addition, we obtained images and spectra for 10 of these PNe, finding low-ionization structures with very different properties relative to each other, in terms of expansion velocities, shapes, sizes and locations relative to the main nebular components. Several physical processes have to be invoked in order to account for the formation and evolution of jets and jet-like structures observed. We present here some results which are illustrative of our work--on IC 4593, NGC 3918, NGC 6337--comparing the different low-ionization structures in these PNe and illustrating some of the questions that we try to address. The most interesting result we obtained is that the collimated LIS present a linear increase in the expansion velocity from inner to outer regions. This is an important constraint on the models and is, in fact, one of the predictions of the interacting-stellar-wind models for the formation of jets, if stellar magnetic fields are considered.