  We present new Arecibo 21-cm line measurements of the temperatures of
interstellar gas. Our temperatures for the Cold Neutral Medium (CNM) are
significantly lower than previous single-dish results and in very good accord
with theoretical models. For warm gas at T > 500 K, we find a significant
fraction of gas (>47%) to lie in the thermally unstable region 500 ---> 5000 K;
moreover, about 60% of all the neutral atomic gas has T > 500 K. Large amounts
of thermally unstable gas are not allowed in theoretical models of the global
interstellar medium.
