As we mentioned in the introduction, the D-branes are characterized by the fact that open strings can end on them. Thus, a Dp-brane is a (p + 1)-dimensional hyperplane spanned by open strings which have the standard Neumann boundary conditions in the (p+1) longitudinal directions and Dirichlet boundary conditions in the remaining (9 — p) transverse directions. In this chapter we are going to present an alternative (though completely equivalent) description based instead on closed strings which are emitted (or absorbed) by world-sheets with boundaries on which the string coordinates obey the appropriate boundary conditions. As we shall see, this description based on the use of the so-called boundary state turns out to be extremely useful for practical applications; moreover it allows us to establish a very clear relation between the stringy description of D-branes to the supergravity description presented in the previous chapter. A more extensive review of this boundary state approach to the D-branes can be found, for example, in , while the standard description based on the use of open strings with Dirichlet boundary conditions can be found in the reviews in .