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We present a tutorial on the determination of the physical conditions andchemical abundances in gaseous nebulae. We also include a brief review ofrecent results on the study of gaseous nebulae, their relevance for the studyof stellar evolution, galactic chemical evolution, and the evolution of theuniverse. One of the most important problems in abundance determinations is theexistence of a discrepancy between the abundances determined with collisionallyexcited lines and those determined by recombination lines, this is called theADF (abundance discrepancy factor) problem; we review results related to thisproblem. Finally, we discuss possible reasons for the large t$^2$ valuesobserved in gaseous nebulae.

Nebular spectroscopy: A guide on H II regions and planetary nebulae