  Dwarf irregulars (dIrrs) are among the most common type of galaxy in the
Universe. They typically have gas-rich, low surface-brightness, metal-poor, and
relatively-thick disks. Here we summarize the current state of our knowledge of
the interstellar medium (ISM), including atomic, molecular and ionized gas,
along with their dust properties and metals. We also discuss star formation
feedback, gas accretion, and mergers with other dwarfs that connect the ISM to
the circumgalactic and intergalactic media. We highlight one of the most
persistent mysteries: the nature of pervasive gas that is yet undetected as
either molecular or cold hydrogen, the ``dark gas''. Here are a few highlights:
  1. Significant quantities of HI are in far-outer gas disks.
  2. Cold HI in dIrrs would be molecular in the Milky Way, making the chemical
properties of star-forming clouds significantly different.
  3. Stellar feedback has a much larger impact in dIrrs than in spiral
galaxies.
  4. The escape fraction of ionizing photons is significant, making dIrrs a
plausible source for reionization in the early Universe.
  5. Observations suggest a significantly higher abundance of hydrogen (H$_2$
or cold HI) associated with CO in star-forming regions than that traced by the
CO alone.
