Mitochondrial replacement therapy (MRT) utilises nuclear transfer technology to replace defective mitochondria with healthy ones and thereby minimise the risk of a mitochondrial disease passing from a mother to her child. It promises much but comes with ethical controversy, significant risk of harm and many unknowns. Forming a position on MRT requires accurate information about the current state of knowledge, and an appreciation of the ethical issues at stake. Ethical deliberations will vary depending on the framework used. There are in (...) principle objections to MRT on the grounds of direct harm to human embryos and germline genetic modification. But even without these objections MRT can be weighed in terms of the balance between risks and benefits, alternatives and uncertainties. This paper explores the evidence and lays out the relevant issues to assist such a deliberation. (shrink)