In this contribution, we present some recent theoretical results concerning
the fringe contrast in Mach-Zehnder atom interferometers and the use of Bl
och states to describe atomic diffraction. We also describe the observation
of diffraction of lithium at thermal energy by a quasi-resonant laser stan
ding wave. (C) 2001 Academie des sciences/Editions scientifiques et medical
es Elsevier SAS.