The seminal 1984 Warnock Report1 established that research on human embryos should be limited to the first 14 days of development (Box 1). Since that time, the rule has been broadly adopted and adhered to across the research community. With the introduction of new methodologies into human embryology, however, our ability to culture human embryos in vitro has progressed rapidly, to the point where we now are reaching the 14-day Rubicon. In August 2016, two groups in the UK and in the US reported experiments on human embryos that were sustained in culture for 12–13 days after fertilization2,3. To comply with British law, the UK lab destroyed its embryo on the 13th day. In the following article, Nature Biotechnology brings together a group of experts to discuss whether, in the light of these advances, it is now time to reassess the 14-day rule.