Effective quiver gauge theories arising from a stack of D3-branes on certain Calabi-Yau singularities can be studied from many points of view. In this talk we adopt a first principle approach via open topological string theory, which means that we construct the natural A- infinity-structure of open string amplitudes in the associated D-brane category. This precisely reproduces the results of the method of brane tilings, without having to resort to any effective field theory computations. In particular, one obtains a general and simple formula for effective superpotentials.