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In this contribution, I will briefly review the development of the theory ofphotoionized gaseous nebulae, highlighting some of the key events. I will thenpresent some recent developments of deep spectroscopy of planetary nebulae(PNe) and H II regions, concentrating on observations of faint heavy elementoptical recombination lines (ORLs). I will show that there is strong evidencethat nebulae contain another previously unknown component of cold (about 1,000K), high-metallicity plasma, probably in the form of H-deficient inclusionsembedded in the warm (about 10,000 K) diffuse nebula of "normal (i.e. nearsolar) composition". This cold gas emits essentially all the observed fluxes ofheavy element ORLs, but is too cool to excite any significant optical orultraviolet collisionally excited lines (CELs) and thus invisible via thelatter. The existence of H-deficient gas in PNe and probably also in H IIregions, not predicted by the current stellar evolution theory, provides anatural solution to the long-standing dichotomy between nebular plasmadiagnostics and abundance determinations using CELs on the one hand and ORLs onthe other, a discrepancy that is ubiquitously observed in Galactic andextragalactic PNe as well as H II regions.

The dark secrets of gaseous nebulae -- highlights from deep spectroscopy