Pre-implantation genetic diagnosis and screening allow couples to avoid having children with life-threatening conditions, but also imply the possibility of ‘screening out’ certain disabilities, raising the prospect of a generation of ‘designer babies’. Can biomedical breakthroughs shape what is to be human? Where does science fact meet science fiction and how can we distinguish between the two? This glimpse of a debate at the Battle of Ideas festival, supported by the Wellcome Trust and bpas delves deeper.