String theory is not only a theory of fundamental one-dimensional strings. There are also a variety of other objects, called branes, of various dimensionalities. The list of possible branes, and their stability properties, depends on the specific theory and vacuum configuration under consideration. One clue for deciphering the possibilities is provided by the spectrum of massless particles. Chapters 4 and 5 described the spectra of massless states that appear in the type I and type II superstring theories in ten-dimensional Minkowski space-time. In particular, it was shown that several antisymmetric tensor (or differential form) gauge fields appear in the R–R sector of each of the type II theories. These tensor fields couple naturally to higher-dimensional extended objects, called D-branes. However, this is not the defining property of D-branes. Rather, the defining property is that D-branes are objects on which open strings can end. A string that does not touch a D-brane must be a closed loop. Those D-branes that have charge couplings to antisymmetric tensor gauge fields are stable, whereas those that do not usually are unstable.

One way of motivating the necessity of D-branes is based on T-duality, so this chapter starts with a discussion of T-duality of the bosonic string theory. Under T-duality transformations, closed bosonic strings transform into closed strings of the same type in the T-dual geometry. The situation is different for open strings, however.