Time and the ephemeral nature of human life have been major themes for poets, philosophers, and theologians. Every scripture, philosophical writing, and work of art addresses, explicitly or implicitly, issues of time and the human experience of it.

Engineers have also considered and often grapple with issues of time, except that, instead of the human experience of it, they deal with the relationship of engineering artifacts with time. Less profound than the previous subject but equally thought-provoking is the transiency, not only of human life, but also of human artifacts. Through structural or functional degradation, or loss of economic relevance, the hand of time lies heavy on engineering designs. Several terms are used to describe this particular aspect of a product or a system's relationship with time, namely the duration from fielding a system, that is, when it first enters operation, to its final breakdown or retirement. These terms include lifespan, service life, durability, and design lifetime, to name a few. This book discusses these issues in the context of complex engineering systems.

More specifically, this book explores an important issue in engineering design that is becoming increasingly critical for complex engineering systems in general, and aerospace systems in particular, namely the selection and implications of a system's design lifetime. Although economists have grappled with the durability choice problem for simple consumer goods, limited attention has been given to the design lifetime problem(s) of complex engineering systems.