Maintainability theory is rapidly growing in importance because of its considerable contribution towards the reduction of maintenance cost of engineering systems during its operational life. At the same time, in order to be used in daily practice, maintainability as a characteristic of engineering systems has to be defined. In technical literature several definitions for maintainability can be found. For example, MIL-STD-721B (1966) defines maintainability as: a characteristic of design and installation which is expressed as the probability that an item will be retained in or restored to a specified condition within a given period of elapsed time, when maintenance is performed in accordance with prescribed procedures and resources. However, in this book the definition proposed by Knezevic (1996) is used: Maintainability is the inherent characteristic of an engineering system related to its ability to be maintained in the State of Functioning by performing the required maintenance tasks as specified. It is necessary to stress that maintainability theory could be a very powerful tool for engineers and managers to quantify and assess the ability of their systems to be maintained in SoFu during operational life.