This paper surveys the literature related to optimal maintenance and replacement models. The emphasis is on work appearing since the 1976 survey, "A Survey of Maintenance Models: The Control and Surveillance of Deteriorating Systems," by W.P. Pierskalla and J .A. Voelker. Thus work done prior to the 1976 survey are not included in this paper. For future theoretical research, possible extensions and open problems are discussed at the end of each section. Furthermore, this paper provides the reader with a quick guide for classifying the literature in maintainability in various ways. Each of the alternative classifications also provides an indication as to the categorization of each of the surveyed articles.
