MAGNETISM ON MEGAPARSEC SCALES Jean Eilek, New Mexico Tech Abstract: We all know ``the nation that controls magnetism will control the universe'' (Diet Smith, from Chester Gould/Dick Tracy). But what do we know today about magnetism on large scales? The largest structures in which we have robust detections of cosmological magnetic fields are clusters of galaxies. We have learned that the plasma atmosphere in every cluster is magnetized and contains relativistic particles (cosmic rays). Because both the fields and the particles need ongoing energization, we know that clusters are not the ``static, spherical chickens'' they were once believed to be; they are dynamic, evolving structures. The evidence suggests that two processes energize the cluster atmosphere today -- radio jets from massive black holes inside the cluster, and the infall of new clumps of matter from outside the cluster.