In this talk, I shall give a brief introduction on gravity driven granular flow through hoppers and silos. Phase transitions of flow on reducing the size of the orifice of the hopper/silo will also be reviewed. When monodisperse grains falling out of a cylindrical silo through a circular exit at the bottom, no continuous flow can be sustained when the diameter D of the exit is about 5 times that of the grains. If the exit of the silo moves horizontally (e.g. circular motion or linear oscillation) with respect to the wall of the silo, finite flow rate can be sustained even at small exit diameter D. If the exit rotates at off-center distance R with angular frequency Omega, the flow rate W increases with R*Omega. Similarly, if the exit oscillates with amplitude A and oscillation frequency f, the flow rate W increases with A*f. On the other hand, if the exit diameter D is large, the flow rate W varies non-monotonically with either the angular frequency Omega or oscillation frequency f. Since silos and hoppers are very common appliances that are used as transporting or distributing granular materials in food, coal and pharmaceutical industries, which suffer high losses due to clogging, our work will not only enhance our basic understanding of the phenomenon of clogging but also improve the design of hoppers and silos for less clogging.