Toby Ord, a philosopher at Oxford University, argues that protecting humanity's long-term future is the most important moral challenge we face. The book examines existential risks, events that could permanently curtail humanity's potential, from natural threats like asteroids and supervolcanoes to human-caused dangers including nuclear war, climate change, pandemics, and artificial intelligence. Ord estimates probabilities for various catastrophic scenarios and finds that total risk over the next century is alarmingly high, driven primarily by technologies we have created ourselves. Drawing on moral philosophy, probability theory, and scientific research, he makes the case that even small reductions in existential risk are enormously valuable given the stakes involved. The book challenges readers to take seriously both the magnitude of what could be lost and the actions that could reduce risk. Ord examines why these issues receive so little attention from governments and philanthropists, proposing institutional changes that could better safeguard humanity's future. He writes as both philosopher and activist, having cofounded the effective altruism movement that seeks to do the most good possible with limited resources. Readers will find here both rigorous analysis and urgent call to action, from someone who believes that what we do in the coming decades will determine whether humanity flourishes for millions of years or destroys itself.