  We investigate the interference correction to the conductivity of a medium
consisting of metallic grains connected by tunnel junctions. Tunneling
conductance between the grains, $e^2g_{\rm T}/\pi\hbar$, is assumed to be
large, $g_{\rm T}\gg 1$. We demonstrate that the weak localization correction
to conductivity exhibits a crossover at temperature $T\sim g^2_{\rm T}\delta$,
where $\delta$ is the mean level spacing in a single grain. At the crossover,
the phase relaxation time determined by the electron-electron interaction
becomes of the order of the dwell time of an electron in a grain. Below the
crossover temperature, the granular array behaves as a continuous medium, while
above the crossover the weak localization effect is largely a single-junction
phenomenon. We elucidate the signatures of the granular structure in the
temperature and magnetic field dependence of the weak localization correction.
