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 \rightarrow 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 .