  Atomic hydrogen (HI) is a critical stepping stone in the gas evolution cycle
of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold,
premolecular state before star formation and the warm, diffuse ISM before and
after star formation. This review describes new, sensitive HI absorption and
emission surveys, which, together with high angular and spectral resolution Hi
emission data, have revealed the physical properties of HI, its structure, and
its association with magnetic fields. We give an overview of the HI phases and
discuss how Hi properties depend on the environment and what its structure can
tell us about feedback in the ISM. Key findings include the following: - The
mass fraction of the cold neutral medium is $\lesssim 40$\% on average,
increasing with $A_V$ due to the increase of mean gas density. - The cold disk
extends to at least $R\sim 25$ kpc. - Approximately 40% of the HI is warm, with
structural characteristics that derive from feedback events. - Cold HI is
highly filamentary, whereas warm HI is more smoothly distributed. We summarize
future observational and simulation opportunities that can be used to unravel
the 3D structure of the atomic ISM and the effects of heating and cooling on HI
properties.
