An issue in engineering design is a system's design lifetime. This book provides a systemic qualitative and quantitative approach to these problems addressing, first, the technicality of durability, second, the marginal cost of durability, and, third, the durability choice problem for complex systems with network externalities (competition and market uncertainty) and obsolescence (technology evolution). Also addressed is the increasing tension between the design lifetimes of complex systems and the shortening time scales associated with the obsolescence of the technology. The book ends with a discussion of flexibility in system design. Dr. Joseph H. Saleh is an Assistant Professor of Aerospace Engineering at the Georgia Institute of Technology. He received his Ph.D. from the Department of Aeronautics and Astronautics at MIT and served as the Executive Director for the Ford-MIT Alliance. His research focuses on issues of design lifetime and how to embed flexibility in the design of complex engineering systems in general and in aerospace system in particular. Dr. Saleh is the author or co-author of 50 technical publications and the recipient of numerous awards for his teaching and research contributions. He served as a technical consultant to NASA's Jet Propulsion Laboratory and has collaborated on research projects with various aerospace companies.