Abstract：Hard x-ray observations provide a unique perspective on the fundamental processes of energy release and transport in solar flares. To obtain such observations hard x-ray imaging spectroscopy can use any of several different indirect modulation techniques. This talk briefly reviews the techniques used by RHESSI (a NASA mission recently decommissioned after 16？ years of operation), STIX (to be launched on Solar Orbiter) and HXI (on ASO-S), with a view to understanding of how their design differences affect the type of science that each can do. In particular the talk reviews the scientific accomplishments of RHESSI, as well as identifying the questions that RHESSI was unable to satisfactorily address. The potential of STIX and HXI to extend our understanding of solar hard X-ray emission will then be considered.