Proper maintenance of plant i terns is crucial for the safe and profi table operation of process plants, The relevant maintenance policies fall into the following four categories: (i) (ii) (iii) (iv) preventivejopportunistic/breakdown replacement policies, inspectionjinspection-repair-replacernent policies, - restorative maintenance policies, and condition based maintenance policies, -For correlating failure times of component equipnent and COmplete systems, the Weibull failure distribution has been used, A new powerful method, SEQLIM, has been proposed for the estimation of the Weibull parameters; particularly, when maintenance records contain very few failures and many successful operation times, When a system consists of a number of replaceable, ageing components, an opporturistic replacernent policy has been found to be cost-effective, A simple opportunistic rrodel has been developed, Inspection models with various objective functions have been investigated, It was fomd that, on the assumption of a negative exponential failure distribution, all models converge to the same optimal inspection interval; provided the safety components are very reliable and the demand rate is low, 'When deterioration becomes a contributory factor to same failures, periodic inspections, calculated from above rrodels, are too frequent, A case of safety trip systems has been studied, A highly effective restorative maintenance policy can be developed if the performance of the equipment under this category can be related to some predictive modelling. A novel fouling model has been proposed to determine cleaning strategies of condensers, Condi tion-based maintenance policies have been investigated. A simple gauge has been designed for condition monitoring of relief valve springs, A typical case of an exothermic inert gas generation plant has been studied, to demonstrate how various policies can be applied to devise overall maintenance actions.