Throughout history, people have sought to improve themselves - and society - by eliminating disease, or enhancing desirable qualities in their children. Others have imposed control over populations with rules about who can marry, who can procreate and who can live. For just over a century, we have referred to this age-old crafting by biological design with a word which became poisonous: eugenics. A guide to an area of science that is little understood but increasingly part of public discourse.