The Mirage Effect or Probe Beam Deflection technique is used to study electrochromic films in two ways. When it results from a concentration gradient, it allows to detect the flux of counter-ions entering or exiting the films. Special attention is given to the case of competitive counter-ions. When it results from a photothermal effect, it allows to measure the coloration/bleaching cycles. Both aspects of the mirage detection are illustrated in reviewing a few examples issued from our laboratory.