  We present the results of the extragalactic surveys performed at 15 and 175
microns using the ISOCAM and ISOPHOT instruments on-board the Infrared Space
Observatory (ISO). The number counts at both wavelengths present a strong
evolution. The preliminary study of the individual galaxies responsible for
this evolution suggest that more star formation was hidden by dust at redshifts
around z=1-2 and imply that the population of luminous IR galaxies detected by
IRAS in the local universe must have played a major role in the formation and
evolution of galaxies and cannot be considered anymore as extreme and rare
cases.
