Until recently, a little less than 20 elements were available for abundance studies in planetary nebulae. These were: H, He, C, N, O, F, Ne, Na, Mg, Si, P, S, Cl, Ar, K, Ca, Mn, Fe, although routine abundance determinations are available for only about 10 elements. As already mentioned and will be made clearer in the next sections, these elements can serve either as probes of the ISM abundances or as probes of the nuclear and mixing processes in the progenitor stars. It has also been mentioned that some elements are heavily depleted in dust grains, so that the abundances of these elements in PNe (Mg, Si, P, K, Ca, Mn, Fe) rather give information on the chemistry of dust grains. This is of great interest since it is now believed that a large portion of grains found in the ISM were actually seeded in the atmospheres of evolved, intermediate mass stars (Dwek 1998).
Recently, ultra deep spectroscopy of bright PNe allowed to detect and measure lines from elements of the fourth, fifth and even sixth row of the Mendeleev table (Péquignot & Baluteau 1994, Baluteau et al. 1995, Dinerstein 2001, Dinerstein & Geballe 2001): V, Cr, Co, Ni, Cu, Zn, Se, Br, Kr, Rb, Sr, Y, Te, I, Xe, Cs, Ba, Pb. When the atomic data for a quantitative analysis of these lines become available (and some atomic physics work has already been fostered by these discoveries, see e.g. Schöning & Butler 1998), this will open a new possibility to test PNe progenitors as production sites of r- and s- process elements.
The determination of isotopic abundance ratios in PNe would allow serious constraints on the nucleosynthesis in post-AGB stars (see e.g. Forestini & Charbonnel 1997, Marigo 2001). They strongly depend on stellar mass, metallicity and mixing length. Unfortunately, from the observational point of view, this field is still in its infancy. The 12C/13C ratio has been measured in only a couple of nebulae in either hyperfine UV transitions (Clegg et al. 1997, Brage et al. 1998) or in millimetric lines of CO (Bachiller et al. 1997, Palla et al. 2000). The 3He abundance has been determined in a few nebulae from the hyperfine transition at 8.665 GHz (Balser et al. 1997, see also Galli et al. 1997) .