A new approach to embryo assessment described at this year’s Annual Meeting of ESHRE may be able to shed light on why so many apparently healthy embryos are not viable. The approach is based on the quantification of mitochondrial DNA found in the outermost layer of cells in a five-day old embryo. The combination of chromosome analysis and mitochondrial assessment may now represent the most accurate and predictive measure of embryo viability with great potential for improving IVF outcome.