After reviewing the general context of complex systems, their engineering and their safety, the book will address the issue of resilience. The objectives of resilience are to face adversity, whatever the dimensions or triggering events of the adversity are, and to keep the system viable, i.e. be able to drive it although with decreased performance. Opposed to safety, resilience will be shown not to be an intrinsic characteristic of the system that could be measured a priori. Paradigms and methods will be introduced to tackle these issues and applications to cybersecurity and enterprise resilience will be discussed.