Nebular spectroscopy: A guide on H II regions and planetary nebulae
  We present a tutorial on the determination of the physical conditions and
chemical abundances in gaseous nebulae. We also include a brief review of
recent results on the study of gaseous nebulae, their relevance for the study
of stellar evolution, galactic chemical evolution, and the evolution of the
universe. One of the most important problems in abundance determinations is the
existence of a discrepancy between the abundances determined with collisionally
excited lines and those determined by recombination lines, this is called the
ADF (abundance discrepancy factor) problem; we review results related to this
problem. Finally, we discuss possible reasons for the large t$^2$ values
observed in gaseous nebulae.
