Gaseous nebulae offer outstanding opportunities to atomic physicists,
spectroscopists, plasma experts, and to observers and theoreticians
alike for the study of attenuated ionized gases. These nebulae are often
dusty, heated by radiation fields and by shocks. They are short-lived
phenomena on the scale of a stellar lifetime, but their chemical
compositions and internal kinematics may give important clues to
advanced stages of stellar evolution. The material herein presented is
based on lectures given at the University of Michigan, University of
Queensland, University of California, Los Angeles, and in more
abbreviated form at the Raman Institute, at the Scuola Internazionale di
Trieste, and elsewhere. Much of it is derived origionally from the
series "Physical Processes in Gaseous Nebulae" initiated at the Harvard
College Observatory in the late 1930s. I have tried to emphasize the
basic physics of the mechanisms involved and mention some of the
uncertainties that underlie calculations of many basic parameters.
Emphasis is placed on ionized plasmas with electron temperatures
typically in the neighborhood of 10, OOOoK. Dust and other ingredients
of the cold component of the interstellar medium are treated briefly
from the point of view of their relation to hot plasmas of H II regions
and planetaries. Chemical composition determinations for nebulae are
discussed in some detail while the last section deals with
interpretations of elemental abundances in the framework of stellar
evolution and nucleogenesis. Gaseous nebulae offer some particularly
engaging opportunities for studies of stellar evolution.