  We are carrying out a study of the physical parameters and excitation of
low-ionization structures (LIS) in planetary nebulae (PNe). Since the optical
morphology and kinematics of the LIS and the main components (rims, shells and
haloes) of our sample were studied previously, our main goal now is to search
for: i) the density contrasts between jets/knots and the main nebular
components; ii) their main excitation processes; and iii) their chemical
abundances. The first results of this survey -- based on the analysis of
NGC7009, NGC6543, NGC6891 and K4-47 -- are that there is no significant density
contrast between LIS and their surroundings, and that most of the LIS studied
(but not all) are mainly photoionized, rather than shock excited.
