The rotor-bearing system of modern rotating machines constitutes a complex dynamic system. The challenging nature of rotordynamic problems have attracted many scientists and engineers whose investigations have contributed to the impressive progress in the study of rotating systems. With the advancement in high-speed machinery and increases in their power/weight ratio, the determination of the rotor dynamic characteristics through reliable mathematical models gain prime importance. The advancement in modern instrumentation and computational capabilities has helped in implementing simulation techniques of these complex models. Modern machinery is bound to fulfill increasing demands concerning durability as well as safety requirements. On-line condition monitoring strategies are becoming increasingly commonplace in a greater range of systems.