My talk will focus on multi-wavelength studies of feedback from active galactic nuclei (AGN). The importance of AGN feedback to the "Magorrian" relation, the cooling flow problem, and to the growth of massive galaxies was realized soon after the Chandra X-ray Observatory was launched. Chandra revealed cavities in the environment of many nearby clusters. The cavities are tracers of a strong coupling between the relativistic plasma in radio sources and the cooling, thermal gas in clusters. They demonstrate clearly that the AGN affects the cooling gas that leads to star formation and galaxy growth. One of the main advantages of the X-ray cavities is that they allow a direct measurement of the bulk of the AGN's power. By measuring the enthalpy of a cavity, and by assuming a timescale for its lifetime (e.g., the buoyancy time, the time required for the cavity to rise buoyantly from the AGN to the location of the cavity), one can estimate the total mechanical power of the AGN independently of the luminous power. Additionally, by using X-ray and radio data, one can derive scaling relation that allow us to estimate the AGN feedback power using only the radio information. I will review the importance of such relations for extending current studies of feedback with upcoming radio telescopes such as LOFAR (and in the future SKA).
