This is the first book on the Which-Is-Better (WIB) Problem. These are questions that in daily life include such as Which is larger, younger and stronger? .

The main objective of this book is summarizing WIB Problems in maintenance and reliability theory. Optimal policies of replacement first, last and overtime are derived and compared theoretically and numerically, and WIB policies are determined. Furthermore, the reliability properties of parallel and standby systems are compared, and WIB system is determined. These WIB Problems are applied to shock and damage models and backup and checkpoint models of computer systems.