After 20 years of operations, the Paranal Observatory has accumulated some experience with maintenance of systems, and has lately adopted the methodology called 'Maintien en Condition Operationnelle' (MCO). We will describe and review the practical implementation of this strategy, the tools used, the benefits and challenges as well as practical examples and how it is overall managed. The approach is also a benchmarking exercise for operation of the ESO-ELT in the future.