  The interstellar medium mediates galactic evolution as the reservoir of
material for future star formation and the repository of energy and matter
output by stellar processes. Canadians have played leading roles in ISM science
for decades. The Canadian Galactic Plane Survey identified a wealth of
small-scale structure in H I emission as well as self-absorption and in the
structure of polarized emission. These observations demonstrated that no phase
of the ISM, including the transition from atomic gas to star formation, can be
understood in isolation. Canadians have also played leading roles in the
characterization of dust with Planck and balloon-borne telescopes. Canadians
have also used pulsar scintillometry to measure structure in the ISM at the
smallest scales, below 1 AU.
  The 2020s offer many opportunities for ISM science in Canada. A major but
cost-effective upgrade to the Synthesis Telescope with broadband (400-1800 MHz)
single-pixel feeds would enable broadband polarimetry as well as wide-area,
arcminute surveys of radio recombination lines. The next generation of
balloon-borne telescopes will investigate magnetic fields and dust properties.
Large single dishes, particularly the Green Bank Telescope, remain essential
for our understanding of the diffuse structure which characterizes the ISM.
Very long baseline interferometry capability enables parallax measurements of
pulsars and masers and for further progress in scintillometry. ISM astronomers
will continue to participate in cosmological experiments including CHIME and
CHORD. Protecting quiet radio frequency interference environments will be ever
more important as broadband observations are ever more central to ISM science.
Computational capability is essential both for numerical work and for handling
the observational data.
