Title: Hot Gas in Galaxy Clusters

Abstract: Clusters of galaxies have substantial amounts of hot
X-ray-emitting gas.  In many clusters, the cooling time of the gas
near the center of the cluster is much shorter than the age of the
cluster.  Yet, there is little evidence that gas actually cools and
drops out of the hot medium.  What keeps the gas hot?  Two ideas are
currently being explored.  One is that mechanical energy from an
active galactic nucleus in a central galaxy supplies the necessary
heat.  The other is that thermal conduction transports heat from the
outer regions of the cluster down to the center.  The talk will review
the current status of the field, focusing on the conduction idea.