ABSTRACT
The D-branes are characterized by the fact that open strings can end on them. This chapter presents 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. The description based on the use of the so-called boundary state turns out to be extremely useful for practical applications. The chapter presents the overlap conditions that the boundary state must satisfy when a constant gauge field is present on the D-brane world-volume. If the constant gauge field is an external magnetic field, the corresponding boundary state describes a stable BPS bound state formed by a Dp-brane with other lower dimensional D-branes.
