Embryonic stem cell research is regulated by different forms of the subsidiarity principle, i.e. research on embryos should only be conducted if no suitable alternatives exist. Four types are discussed: animal versus human material, adult versus embryonic stem cells, affected or at risk embryos versus healthy embryos, and supernumerary versus research embryos. Three major arguments regarding the subsidiarity principle are discussed: the necessity argument, the least offensive moral approach and the 'nothing is lost' argument. It is proposed that the burden of proof should be shifted onto those who oppose embryonic research. When the freedom of research and the moral obligation to relieve human suffering is taken seriously, the opponents of this research should first demonstrate that embryonic stem cells do not work or that adult stem cells work better.