  As we strive to understand how galaxies evolve it is crucial that we resolve
physical processes and test emerging theories in nearby systems that we can
observe in great detail. Our own Galaxy, the Milky Way, and the nearby
Magellanic Clouds provide unique windows into the evolution of galaxies, each
with its own metallicity and star formation rate. These laboratories allow us
to study with more detail than anywhere else in the Universe how galaxies
acquire fresh gas to fuel their continuing star formation, how they exchange
gas with the surrounding intergalactic medium, and turn warm, diffuse gas into
molecular clouds and ultimately stars. The $\lambda$21-cm line of atomic
hydrogen (HI) is an excellent tracer of these physical processes. With the SKA
we will finally have the combination of surface brightness sensitivity, point
source sensitivity and angular resolution to transform our understanding of the
evolution of gas in the Milky Way, all the way from the halo down to the
formation of individual molecular clouds.

Astrophysics of Galaxies