Abstract: A method of modelling a maintenance system, the method comprising; providing a computer system having a component database and a reliability database; inputting, to the component database, component data relating to the status of one or more components to be maintained as part of the maintenance system; inputting, to the reliability database, reliability data relating to the predicted performance of at least one component represented by the component data in the component database; and computing, in accordance with a predefined relationship between the component and reliability data, output data representing a maintenance prediction relating to said at least one of the components.