Cavities and shock fronts created by radio jets advancing into hot 
atmospheres  of galaxies and clusters provide a reliable measure of 
the energetics of radio sources.
High resolution X-ray and radio observations show that even modest 
radio sources can be extraordinarily powerful. Periodic AGN activity, 
possibly controlled by feedback, supplies enough energy to suppress 
cooling flows in many systems.
Cluster radio lobes are generally out of equipartition, and they 
exhibit large variations in radiative efficiency.   Synchrotron ages 
correlate poorly with buoyancy ages estimated independently by X-ray 
observations, indicating perhaps that the dynamics of these systems 
may be more complicated than we might have hoped.