As the wind turbine capacity ratings continue to rise and the wind farm locations move further offshore, a great deal of attention is focused on reducing operation and maintenance (O&M) costs whilst ensuring the highest levels of reliability and safety. Currently, the O&M costs (including expenditures related to the planned and unplanned repairs of units or systems) constitute a substantial part of the cost of energy produced by offshore wind turbines. However, it has been reported in several large-scale wind energy projects (e.g. Opti-OWECS) that a significant portion of annual budget is wasted as a result of unnecessary or improperly carried out maintenance activities. In recent years, wind energy operators have applied different models and methods to maintenance scheduling and optimization to determine the most effective and efficient inspection/repair action policies for wind farm assets so that the best possible balance between reliability targets and operating expenses is achieved. This paper provides an overview of the maintenance scheduling and optimisation models and methods used in wind parks. We will also discuss some new topics for future research.