The physical and abundance properties of the gaseous components of planetary nebulae have traditionally been studied using their rich, pan-chromatic emission line spectra. However, absorption lines produced within the column of nebular gas in front of the central star can provide complementary information, giving access to ions and energy levels that cannot be studied via emission lines and yielding new evidence on the presence and nature of internal inhomogeneities. The far ultraviolet (UV) spectral region, as studied with HST and FUSE, is particularly useful for probing species in the photodissociation region (PDR), such as H$_2$ and O I. Comparison of absorption-line and emission line results within the same nebula indicates strong spatial segregation of the molecular material – and possibly of the dust as well – globally and/or on small physical scales. Measurements of UV absorption lines from excited fine-structure levels of O I reveal that non-thermal (e.g. fluorescent) processes can have an important influence on the level populations, affecting common interpretations of the important infrared cooling lines at 63 and 145 $\mu$m. Unsaturated absorption lines usable for deriving column densities and their ratios can be seen from excited fine-structure levels of some ions abundant in the ionized zone, and in some cases from ions of rarer elements such as the neutron-capture element Ge, which can be self-enriched due to nuclear processing in the PN progenitor star.