Recent radio and X-ray data from radio sources in galaxy clusters are used to place constraints on the particle content and kinematics of jets that supply these radio sources. These data show that the pdV work required to inflate the radio lobes in the ICM exceeds all other estimates of the amount of energy needed to supply the observed radio emission. If the required jet energy density has an isotropic pressure, then in almost all cases the jets cannot be confined by the external pressure of the ICM. This problem can be resolved with jets dominated by a cold and relatively dense proton population, but even here the accompanying energy density in electrons alone can cause jet decollimation in many cases. Calculation of particle interactions in a cold proton jet shows the electron-proton energy transfer times to be very long. Electron-positron jets, unless highly beamed and with an unusual energy distribution, cannot solve the decollimation problem. A viable alternative may be ``Poynting flux''-dominated jets with a very low particle content.
Galaxies: Clusters: General - Galaxies: Jets