We propose to study the near, mid and far-IR variability in two well known Blazers. The light curves will be monitored on timescales ranging from minutes to months with CAM and PHT multiband photometry. One of the sources will be observed simultaneously in X-rays. Our aims are to: - identify the characteristic timescale of variability at these wavelengths. - search for evidence of spectral changes during flares - investigate the possible role of the far-IR as seed photons for the X and gamma-ray component.