In this talk we will discuss three loosely related topics: the geometric construction of fractional branes in C^d/Z_n orbifolds, the problem of establishing the world-volume theory of a space-filling D3-brane at a toric singularity, and the proof of the orbifold/Landau-Ginzburg quantum symmetry in the context of derived categories. We will see that in all three cases the use of stacks is indispensable.