The American development and deployment of Fifth-Generation stealth aircraft like the F-35 Lightning is one of the central stories of today’s security zeitgeist. But behind the scenes, several countries are already looking ahead to the design of a Sixth-Generation jet. The relentless pace of research is arguably driven less by combat experience—of which there is little—and more by a sober assessment that development of a successor will take multiple decades and is better started sooner rather than later. The Sixth-Generation fighter developers can be divided into two categories: the United States, which has developed and deployed two stealth fighter types, and countries that have skipped or given up on their attempt to build Fifth Generations jets. These latter countries have concluded that doing so is so time-consuming and expensive that it makes more sense to focus on tomorrow's technology than try to catch up with today's.