  We study the construction of D-brane boundary states in the pure spinor
formalism for the quantisation of the superstring. This is achieved both via a
direct analysis of the definition of D-brane boundary states in the pure spinor
conformal field theory, as well as via comparison between standard RNS and pure
spinor descriptions of the superstring. Regarding the map between RNS and pure
spinor formulations of the superstring, we shed new light on the tree level
zero mode saturation rule. Within the pure spinor formalism we propose an
explicit expression for the D-brane boundary state in a flat spacetime
background. While the non-zero mode sector mostly follows from a simple
understanding of the pure spinor conformal field theory, the zero mode sector
requires a deeper analysis which is one of the main points in this work. With
the construction of the boundary states at hand, we give a prescription for
calculating scattering amplitudes in the presence of a D-brane. Finally, we
also briefly discuss the coupling to the world-volume gauge field and show that
the D-brane low-energy effective action is correctly reproduced.
