 “Parenthood” has distinct senses: biological, social, legal, and moral. These categories present problems at the margins. While the idea of a biological parent seems self-evident, modern reproductive technology complicates it, as a child can have genetic parents (gamete providers, who supply the sperm or egg) and a third, gestational, parent. Each of these is a biological parent, by virtue of making a biological contribution to producing the child. The recent technology of mitochondrial replacement therapy, in which the nucleus of a fertilized egg is transferred to a second egg for medical reasons, introduces further complications, as additional genetic material—mitochondrial DNA—is supplied by the donor egg cell.