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The importance of spin temperature for 21-cm line studies is reviewed, and four mechanisms which affect it are studied. Two of the mechanisms, collisions with free electrons and interactions with light, are studied here in detail for the first time. The results are summarized in Table II of Section VI, in the form of certain efficiencies which can be used with (15) to calculate the spin temperature. In Section VI the results are applied to a variety of astronomical situations, and it is shown that in the usual situation collisions with H atoms are very effective in establishing the spin temperature equal to the kinetic temperature. Under conditions of low-density and/or high-radiation intensity, however, important deviations from the usual are noted. The significance of such deviations for absorption studies of radio sources and the galactic halo is discussed. In Section VII the deuterium line at 91.6 cm is considered in like fashion. It is shown that for deuterium also, the spin temperature probably is close to the kinetic temperature.