The maintenance of a deteriorating system is often imperfect: the system after maintenance will not as good as new, but younger. Imperfect maintenance study indicates a significant breakthrough in reliability and maintenance theory. Research activities in maintenance engineering have been conducted over the past 30 years and more than 40 mathematical imperfect maintenance models have been proposed for estimating the reliability measures and determining the optimum maintenance policies. Various treatment methods and optimal policies on the imperfect maintenance are discussed and summarized in this paper. Several important results for imperfect maintenance are also presented.