  Two aspects of the contribution of grazing incidence fast atom diffraction
(GIFAD) to molecular beam epitaxy (MBE) are reviewed here: the ability of GIFAD
to provide \emph{in-situ} a precise description of the atomic-scale surface
topology, and its ability to follow larger-scale changes in surface roughness
during layer-by-layer growth. Recent experimental and theoretical results
obtained for the He atom beam incident along the highly corrugated $[ 1\bar{1}0
]$ direction of the $\beta_{2}$(2$\times$4) reconstructed GaAs(001) surface are
summarized and complemented by the measurements and calculations for the beam
incidence along the weakly corrugated [010] direction where a periodicity twice
smaller as expected is observed. The combination of the experiment, quantum
scattering matrix calculations, and semiclassical analysis allows in this case
to reveal structural characteristics of the surface. For the in situ
measurements of GIFAD during molecular beam epitaxy of GaAs on GaAs surface we
analyse the change in elastic and inelastic contributions in the scattered
beam, and the variation of the diffraction pattern in polar angle scattering.
This analysis outlines the robustness, the simplicity and the richness of the
GIFAD as a technique to monitor the layer-by-layer epitaxial growth.
