Ionized nebulae have been targets of interest since the introduction of the telescope
centuries ago. These isolated, "classical" H II regions gave us some of the earliest
insight into the copious feedback energy that stars inject into the interstellar medium.
Their unique spectra contain information about the quality and quantity of the ionizing
field as well as the temperature, density, and metallicity of these discrete locations
in the Galaxy. With increasing sensitivity across many spectral domains, we now know
that ionized gas is not localized to massive star regions in many star-forming galaxies.
In particular, recent observational studies allow a thorough comparison of the physical
conditions and distribution of the well-studied classical H II regions to the more
widespread warm, diffuse gas. By more realistically evolving a dynamic interstellar
medium, models are beginning to reproduce the observed emission measure variations
and provide a natural solution to the propagation of ionizing flux from a predominantly
neutral galactic disk to the distant halo.