  Recent observations of high energy (> 20 keV) X-ray emission in a few
clusters extend and broaden our knowledge of physical phenomena in the
intracluster space. This emission is likely to be nonthermal, probably
resulting from Compton scattering of relativistic electrons by the cosmic
microwave background radiation. Direct evidence for the presence of
relativistic electrons in some 30 clusters comes from measurements of extended
radio emission in their central regions. I first review the results from RXTE
and BeppoSAX measurements of a small sample of clusters, and then discuss their
implications on the mean values of intracluster magnetic fields and
relativistic electron energy densities. Implications on the origin of the
fields and electrons are briefly considered.
