The search of distortions (deviations from a Planck distribution) in the spectrum of the Cosmic Microwave Background allows to probe the conditions existing in our Universe up toZ≃106.We analyze the status of the observations (so far no distortion has been detected) and discuss the possibility and the astrophysical interest of searching distortions at long wavelengths, in the Rayleigh Jeans portion of the Cosmic Microwave Background spectrum. ©1999 American Institute of Physics.