  We observed six fields of the Small Magellanic Cloud (SMC) with the Advanced
Camera for Survey on board the Hubble Space Telescope in the F555W and F814W
filters. These fields sample regions characterized by very different star and
gas densities, and, possibly, by different evolutionary histories. We find that
the SMC was already forming stars ~12 Gyr ago, even if the lack of a clear
horizontal branch suggests that in the first few billion years the star
formation activity was low. Within the uncertainties of our two-band
photometry, we find evidence of a radial variation in chemical enrichment, with
the SMC outskirts characterized by lower metallicity than the central zones.
  From our CMDs we also infer that the SMC formed stars over a long interval of
time until ~2-3 Gyr ago. After a period of modest activity, star formation
increased again in the recent past, especially in the bar and the wing of the
SMC, where we see an enhancement in the star-formation activity starting from
~500 Myr ago. The inhomogeneous distribution of stars younger than ~100 Myr
indicates that recent star formation has mainly developed locally.
