  The evolution of radial gradients of metallicity in disk galaxies and its
relation with the disk formation are not well understood. Theoretical models of
galactic chemical evolution make contrasting predictions about the time
evolution of metallicity gradients. To test chemical evolution models and trace
the star formation and accretion history of low luminosity disk galaxies we
focus on the Local Group galaxy M33. We analyze O/H and S/H abundances in
planetary nebulae, H{\sc ii} regions, and young stars, together with known
[Fe/H] abundances in the old stellar population of M33. With a theoretical
model, we follow the time evolution of gas (diffuse and condensed in clouds),
stars, and chemical abundances in the disk of M33, assuming that the galaxy is
accreting gas from an external reservoir. Our model is able to reproduce the
available observational constraints on the distribution of gas and stars in M33
and to predict the time evolution of several chemical abundances. In
particular, we find that a model characterized by a continuous infall of gas on
the disk, at a rate of $\dot M_{\rm inf}\approx 1$ $M_\odot$ yr$^{-1}$, almost
constant with time, can also account for the relatively high rate of star
formation and for the shallow chemical gradients. Supported by a large sample
of high resolution observations for this nearby galaxy, we conclude that the
metallicity in the disk of M33 has increased with time at all radii, with a
continuous flattening of the gradient over the last $\sim 8$ Gyr.
