BPS states and algebras from quivers

We discuss several aspects of D-brane moduli spaces and BPS spectra near orbifold points. We give a procedure to determine the decay products on a line of marginal stability, and we define the algebra of BPS states in terms of quivers. These issues are illustrated in detail in the case of type IIA theory on C^2/Z_N. We also show that many of these results can be extended to arbitrary points in the compactification moduli space using Pi-stability.