Systems are designed to fulfill actual or perceived needs. The identification of a need, a functional deficiency, triggers the definition of the need in the form of high-level requirements, enabling the identification of potential design concepts, which results in the selection of a preferred solution. The definition of a selected design concept is accomplished through detailed requirements, and subsequent design and development of the system which satisfies those requirements effectively and efficiently throughout the intended in-service life. The design and development process can easily span a number of years. This may result in evolved or depreciated requirements by the time that the systems is commissioned and enters into service, and the system may no longer satisfy in full the current need. Even if the system fully satisfies the user needs, the operational life of the system will inevitably result in a degradation in performance. No matter the effort deployed in maintenance programs and even in reliability growth programs, eventually reliability (whether the system has been in active use or even in a stand-by mode) takes a toll in system performance. Long gone are the days in which failure rates were assumed to be constant [Wong, 1981; McLinn, 1989]; increasing failure rates imply, among other things, less operational availability and thus lower perform!