ABSTRACT
This chapter shows how the low-energy effective action of a D-brane is related to the boundary state. The boundary state is the exact conformal description of a D-brane and therefore it contains the complete information about the interactions between a D-brane and the closed strings that propagate in the bulk. In particular, it encodes the couplings with the bulk massless fields which can be simply obtained by saturating the boundary state with the massless states of the closed string spectrum. The chapter explicitly shows that by projecting the boundary state with an external field onto the massless states of the closed string spectrum, one can reconstruct the linear part of the low-energy effective action of a Dp-brane.
