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