The intragalactic medium (IGM) in clusters and groups of galaxies is an important large-scale feature of the universe. X-ray observations show us that this plasma, which exists on a scales up to many Megaparsecs, has a significant hot component. Radio observations show us that relativistic particles and magnetic fields can be another important component of the medium in some situations.
The medium is important in its own right as it offers an extreme environment in which to study plasma processes. How ubiquitous are magnetic fields in the IGM? How were they formed? Are dynamo processes responsible? What rôle do they play in so called ``cooling flows''? Is the energy density of magnetic fields important relative to thermal gas? How are relativistic particles accelerated and/or re-accelerated on large scales? Are they energetically important? What is the range of temperatures in the thermal IGM and how is it regulated?
But it is also becoming clear that infall of individual galaxies and
small groups into bigger clusters and more equal cluster-cluster
mergers are both important for galaxy evolution. Studies of clusters
at 
0.4 using the Hubble Space Telescope show that galaxy
morphology has changed with epoch. The most obvious change over this
period is the size of the clusters which have been merging. The
further back we look, the smaller the building blocks should be. The
rate of such mergers depends on the cosmological density parameter:
the number of recent mergers to be expected increases with the density
of the universe. The most important changes in the clusters are
expected to take place in the IGM. The density of the IGM will, in
turn, modify galaxy properties as they pass through it. The system of
gas and galaxies is coupled.
The enhanced VLA will be able to study the properties of the IGM in various ways. The nonthermal radio halos found in some rich dense clusters trace a combination of the relativistic particles and the perpendicular magnetic fields. Faraday rotation by the medium between us and the radio sources probes the line-of-sight magnetic field and the thermal electron density. The Sunyaev-Zeldovich effect probes an integral of the thermal pressure along each line of sight through a cluster. The properties of twin jet radio galaxies also probe pressures and pressure gradients in the IGM. Finally, gravitational lenses can be used to estimate the mass along the line-of-sight through rich clusters.