The baryons in Clusters of Galaxies are mostly in the form of a hot diffuse gas with a temperature of 10^7 to 10^8K, depending on the total cluster mass. The radiative cooling time of the hot gas is less than a billion years in the cores of many clusters which, in the absence of any heat source, would lead to large cooling rates and consequent star formation. This is rarely seen. Energy injected into the gas from accretion onto the central black hole prevents widespread cooling through the process of AGN Feedback. How this process operates will be discussed and illustrated using data from the Chandra, XMM and Hitomi observatories.