The Interstellar Medium: The Key Component in Galactic Evolution and
  Modern Cosmology
  The gases of the interstellar medium (ISM) possess orders of magnitude more
mass than those of all the stars combined and are thus the prime component of
the baryonic universe. With L-band surface sensitivity even better than the
planned phase one Square-Kilometer-Array (SKA1), the Five-hundred-meter
Aperture Spherical radio Telescope (FAST) promises unprecedented insights into
two of the primary components of ISM, namely, the atomic hydrogen (HI) and the
hydroxyl molecule (OH). We discuss here the evolving landscape of our
understanding of ISM, particularly, its complex phases, the magnetic fields
within, the so-called dark molecular gas (DMG), high velocity clouds, and the
connection between local and distant ISM. We lay out, in broad strokes, several
expected FAST projects, including an all northern-sky high-resolution HI survey
(22,000 deg$^2$, 3\arcmin\ FWHM beam, 0.2 km/s), targeted OH mapping, searching
for absorption or masing signals, and etc. Currently under commissioning, the
commensal observing mode of FAST will be capable of simultaneously obtaining HI
and pulsar data streams, making large-scale surveys in both science areas more
efficient.
