  We have derived the star formation history (SFH) of the blue compact dwarf
galaxy IZw18 through comparison of deep HST/ACS data with synthetic color
magnitude diagrams. A statistical analysis was implemented for the
identification of the best-fit SFH and relative uncertainties. We confirm that
IZw18 is not a truly young galaxy, having started forming stars earlier than ~1
Gyr ago, and possibly at epochs as old as a Hubble time. In IZw18's main body
we infer a lower limit of ~2 x 10^{6} M_sun for the mass locked-up in old
stars. IZw18' s main body has been forming stars very actively during the last
~10 Myr, with an average star formation rate (SFR) as high as ~1 M_sun/yr (or
~2 x 10^{-5} M_sun yr^{-1} pc^{-2}). On the other hand, the secondary body was
much less active at these epochs, in agreement with the absence of significant
nebular emission. The high current SFR can explain the very blue colors and the
high ionized gas content in IZw18, resembling primeval galaxies in the early
Universe. Detailed chemical evolution models are required to quantitatively
check whether the SFH from the synthetic CMDs can explain the low measured
element abundances, or if galactic winds with loss of metals are needed.
