Gene editing with CRISPR technology is transforming biology. Understanding the underlying chemical mechanisms of RNA-guided DNA and RNA cleavage provides a foundation for both conceptual advances and technology development. Professor Jennifer Doudna ForMemRS discusses how bacterial CRISPR adaptive immune systems inspire creation of powerful genome engineering tools, enabling advances in both fundamental biology and applications in medicine. She will also discuss the ethical challenges of some of these applications. The Croonian Medal Prize Lecture is the premier lecture in the biological sciences and is delivered annually at the Royal Society in London.