Physical properties of the atomic gas in spiral galaxies are briefly considered. Although both Warm (WNM, 104 K) and Cool (CNM, ∼ 100 K) atomic phases coexist in many environments, the dominant mass contribution within a galaxy's star-forming disk (R25) is that of the CNM. Mass fractions of 60 to 90% are found within R25. The CNM is concentrated within moderately opaque filaments with a face-on surface covering factor of about 15%. The kinetic temperature of the CNM increases systematically with galactocentric radius, from some 50 to 200 K, as expected for a radially declining thermal pressure in the galaxy mid-plane. Galaxies of different Hubble type form a nested distribution in TK(R), apparently due to the mean differences in pressure which result from the different stellar and gas surface densities. The opaque CNM disappears abruptly near R25, where the low thermal pressure can no longer support the condensed atomic phase. The CNM is apparently a prerequisite for star formation. Although difficult to prove, all indications are that at least the outer disk and possibly the inter-arm atomic gas are in the form of WNM, which accounts for about 50% of the global total. Median line profiles of the CNM display an extremely narrow line core (FWHM ∼ 6 km s−1) together with broad Lorentzian wings (FWHM ∼ 30 km s−1). The line core is consistent with only opacity broadening of a thermal profile.