The greatest existential risks over the coming decades or century arise from certain, anticipated technological breakthroughs that we might make in particular, machine super intelligence, nanotechnology and synthetic biology. Each of these has an enormous potential for improving the human condition by helping cure disease, poverty, etc. But one could imagine them being misused, used to create powerful weapon systems, or even some kind of accidental destructive scenario, where we suddenly are in possession of some technology that's far more powerful than we are able to control or use wisely.