  I review what is known about the temperature of the plasma within stellar
wind bubbles and superbubbles. Classical theory suggests that it should be hot,
with characteristic temperatures of order a million degrees. This temperature
should be set by the balance between heating by the internal termination shocks
of the central stellar winds and supernovae, which expand at thousands of km/s,
and cooling by conductive evaporation of cold gas off the shell walls. However,
if the hot interior gas becomes dense enough due to evaporation or ablation off
of interior clouds, it will cool in less than a dynamical time, leading to a
cold interior. The observational evidence appears mixed. On the one hand, X-ray
emission has been observed from both stellar wind bubbles and superbubbles. On
the other hand, no stellar wind bubble or superbubble has yet been observed
emitting at the rate predicted by the classical theory: they are either too
faint or too bright, by up to an order of magnitude. Alternate explanations
have been proposed for the observed emission, including off-center supernova
remnants hitting the shell walls of superbubbles, and residual emission from
highly-ionized gas out of coronal equilibrium. Furthermore, the structures of
post-main sequence stellar wind bubbles, expanding into what are presumably old
stellar wind bubbles, appear in at least some cases to show that the bubble
interior is cold, not hot. (The classical example of this is NGC 6888.) What is
the actual state of bubble and superbubble interiors?
