More particularly, we will compare condition-based maintenance and age-based replacement (ABR) in the context of opportunistic maintenance, whereby maintenance may or may not be ideal. We specifically compare CBM to ABR since ABR is widely used in practice and more efficient than other preventive maintenance approaches (such as block replacement). In ABR, components are replaced after a fixed time interval or at the occurrence of a failure, whichever occurs first (Marquez 2007). Any unpredicted failure postpones the next scheduled maintenance activity. The concepts of opportunistic maintenance and ideal (and non-ideal) maintenance are explained in the next two paragraphs