This paper by Joseph Polchinski discusses Dirichlet-branes (D-branes) in type II string theory. D-branes are extended objects defined by mixed Dirichlet-Neumann boundary conditions and break half of the supersymmetries of the type II superstring. They carry a complete set of electric and magnetic Ramond-Ramond (RR) charges. The product of the electric and magnetic charges is a single Dirac unit, and the quantum of charge matches the value required by string duality. This provides strong evidence that D-branes are intrinsic to type II string theory and are the RR sources required by string duality. The paper also notes the existence of a previously overlooked 9-form potential in the IIa string, which gives rise to an effective cosmological constant of undetermined magnitude. The NSNS and RR gauge fields are quite different in perturbation theory. String states carry the world-sheet charge associated with the NSNS gauge symmetry, while they are neutral under the RR symmetries. The NSNS gauge fields are well-studied in conformal field theory, whereas the RR gauge fields are not easily understood in this way. String duality suggests that such world-sheet distinctions are artifacts of string perturbation theory. Various dualities interchange NSNS and RR states, and string duality requires that states carrying various RR charges exist. Previously, these were suggested to be black p-branes. In this paper, it is shown that D-branes are another class of objects that carry RR charges. The paper discusses the properties of D-branes, their relation to RR charges, and the implications of string duality. It also considers the consistency of D-branes in type II string theory and their relation to the type I theory. The paper concludes that D-branes are the RR-charged objects required by string duality, and that their charges satisfy the Dirac quantization condition. The paper also discusses the implications of the 10-form potential in the IIa string and its effect on the cosmological constant. The paper concludes that the 10-form potential is a violation of two pieces of string lore, but it is possible that its value will turn out to be quantized in string units.This paper by Joseph Polchinski discusses Dirichlet-branes (D-branes) in type II string theory. D-branes are extended objects defined by mixed Dirichlet-Neumann boundary conditions and break half of the supersymmetries of the type II superstring. They carry a complete set of electric and magnetic Ramond-Ramond (RR) charges. The product of the electric and magnetic charges is a single Dirac unit, and the quantum of charge matches the value required by string duality. This provides strong evidence that D-branes are intrinsic to type II string theory and are the RR sources required by string duality. The paper also notes the existence of a previously overlooked 9-form potential in the IIa string, which gives rise to an effective cosmological constant of undetermined magnitude. The NSNS and RR gauge fields are quite different in perturbation theory. String states carry the world-sheet charge associated with the NSNS gauge symmetry, while they are neutral under the RR symmetries. The NSNS gauge fields are well-studied in conformal field theory, whereas the RR gauge fields are not easily understood in this way. String duality suggests that such world-sheet distinctions are artifacts of string perturbation theory. Various dualities interchange NSNS and RR states, and string duality requires that states carrying various RR charges exist. Previously, these were suggested to be black p-branes. In this paper, it is shown that D-branes are another class of objects that carry RR charges. The paper discusses the properties of D-branes, their relation to RR charges, and the implications of string duality. It also considers the consistency of D-branes in type II string theory and their relation to the type I theory. The paper concludes that D-branes are the RR-charged objects required by string duality, and that their charges satisfy the Dirac quantization condition. The paper also discusses the implications of the 10-form potential in the IIa string and its effect on the cosmological constant. The paper concludes that the 10-form potential is a violation of two pieces of string lore, but it is possible that its value will turn out to be quantized in string units.