Given the uncertainties in the derivation of the current and past SFR in galaxies of any kind, the SF regime in irregulars is subject of wide debate. Since the most direct information on the stellar populations and relative histories in any system can be derived from their CMD, we have undertaken a project for accurately studying the CMDs and luminosity functions (LF) of nearby, well resolved, irregulars. The method proceeds on two tracks: a) we have developed a numerical code for Montecarlo simulations of CMDs and b) we have taken deep and accurate CCD photometry of several galaxies and derived the corresponding CMDs and LFs. The comparison of the observational data with the corresponding theoretical expectations provides several constraints on the SF history and on the IMF of the analysed objects. We do not pretend to reach unique conclusions on the evolution of irregular galaxies, but we can sensibly reduce the range of possible interpretations.
All the irregulars in our sample were selected from the DDO Catalogue and were supposed to belong to the Local Group. Detailed descriptions of the data acquisition and reduction and of the simulation code can be found in Tosi et al. (1991). In the following we briefly summarize the results relative to Sextans B, NGC 3109 and DDO 210.