The scientific performances of future hard X-ray missions will necessitate a very low detector background level. This will imply thorough background simulations, and efficient background rejection systems. It necessitates also a very good knowledge of the detectors to be shielded. We got experience on these activities by conceiving and optimizing the active and passive background rejection system of the Simbol-X and IXO/HXI missions. Considering that this work may naturally be extended to other X-ray missions, we have initiated with CNES, in 2010, a R&T project on the study of background rejection systems, whose status will be presented in this paper.