Abstract: An accurate knowledge of all phases of the interstellar medium and physical processes is critical for understanding the triggering and cessation of star formation and galaxy evolution. Warm ionized gas can be detected with optical emission lines and have been studied for decades. However, there are two classes of widespread optical line-emitting regions for which the ionization mechanisms are still evading us 30 years after their discoveries: low ionization emission (LINER-like) in quiescent galaxies and the diffuse ionized gas (DIG) in star-forming galaxies. They may hold critical clues to the heating mechanisms of the ISM, and the evolution of ISM as star formation shuts down. I will discuss the progress we are making on these issues, specifically about the ionization mechanism for LINER-like objects, the potential connection between LINER-like gas and DIGs, and their elemental abundance patterns.