Edward Witten discusses the existence of bound states of strings and p-branes in the context of Type II superstrings. The recent discovery of an explicit conformal field theory description of Type II p-branes allows for the investigation of their bound states. In particular, it has been verified that the Type IIB superstring in ten dimensions has a family of soliton and bound state strings permuted by SL(2,Z). The space-time coordinates enter the formalism as non-commuting matrices. The paper explores the structure of D-branes and their relation to the Type IIB superstring. It shows that D-strings have the same low-lying world-sheet excitations as the fundamental Type IIB superstring and can be interpreted as the (0,1) string. The paper also discusses the existence of bound states of m fundamental strings with a D-string, showing that the binding energy is almost one hundred per cent. This is due to the fact that the tension of an (m,n) string is given by T_{m,n} = T sqrt(m² + n²/λ²), where T is a constant and λ is the string coupling constant. The paper also discusses the general setting for analyzing bound states of D-branes and applies it to D-strings. It shows that the low energy theory on the world-volume of n parallel D-branes is the dimensional reduction of ten-dimensional supersymmetric Yang-Mills theory with gauge group U(n). The paper also discusses the mass gap in supersymmetric gauge theories and the implications for bound states of D-branes. The paper concludes by discussing the bound states of other D-branes and p-branes, including instantons, three-branes, five-branes, and seven-branes. It shows that the existence of bound states of these objects is predicted by SL(2,Z) symmetry and that the predictions are consistent with the known properties of these objects in string theory. The paper also discusses the implications of these results for the understanding of string theory and the behavior of p-branes in various dimensions.Edward Witten discusses the existence of bound states of strings and p-branes in the context of Type II superstrings. The recent discovery of an explicit conformal field theory description of Type II p-branes allows for the investigation of their bound states. In particular, it has been verified that the Type IIB superstring in ten dimensions has a family of soliton and bound state strings permuted by SL(2,Z). The space-time coordinates enter the formalism as non-commuting matrices. The paper explores the structure of D-branes and their relation to the Type IIB superstring. It shows that D-strings have the same low-lying world-sheet excitations as the fundamental Type IIB superstring and can be interpreted as the (0,1) string. The paper also discusses the existence of bound states of m fundamental strings with a D-string, showing that the binding energy is almost one hundred per cent. This is due to the fact that the tension of an (m,n) string is given by T_{m,n} = T sqrt(m² + n²/λ²), where T is a constant and λ is the string coupling constant. The paper also discusses the general setting for analyzing bound states of D-branes and applies it to D-strings. It shows that the low energy theory on the world-volume of n parallel D-branes is the dimensional reduction of ten-dimensional supersymmetric Yang-Mills theory with gauge group U(n). The paper also discusses the mass gap in supersymmetric gauge theories and the implications for bound states of D-branes. The paper concludes by discussing the bound states of other D-branes and p-branes, including instantons, three-branes, five-branes, and seven-branes. It shows that the existence of bound states of these objects is predicted by SL(2,Z) symmetry and that the predictions are consistent with the known properties of these objects in string theory. The paper also discusses the implications of these results for the understanding of string theory and the behavior of p-branes in various dimensions.