Corrective maintenance includes the unscheduled repairs required to return a structure from a failed state to an operating or available state. Corrective maintenance is determined by the reliability of the structure. Preventive maintenance includes scheduled action to retain a structure in an operational or available state by preventing failures from occurring. Preventive maintenance directly affects reliability. The trade-offs between the costs for corrective and preventive maintenance are shown. An economic objective to determine the optimum level of maintenance would be to determine the total minimum cost. This objective is discussed in the paper, along with other aspects of the subject.