Hi: Temperatures, Magnetism, Turbulence, Morphology

We review observations of the physical properties of the diffuse ISM HI components, namely the Cold and Warm Neutral Media (CNM and WNM). There is somewhat more WNM than CNM, and at least half of the WNM is not thermally stable. The CNM has typical turbulent Mach number 3. Magnetic energy densities in the CNM considerably exceed thermal energy densities, and are comparable to turbulent energy densities. Magnetic fields in the CNM are comparable to those in the Warm Ionized Medium and the volume-average field strength from synchrotron emissivity.