Abstract: An accurate knowledge of all phases of the instellar medium and physical processes are 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, I will highlight some results on this topic based on the spatially-resolved spectroscopy data from the SDSS-IV MaNGA survey, which is an integral field spectroscopy survey of 10,000 nearby galaxies. I will also discuss the general science goals of the MaNGA, and aspects of the survey design and data products.