Operation and maintenance (O&M) are significant contributors to the cost of energy in wind industry. To reduce the cost, effective maintenance strategies have become an indispensable part of operational decision-making for wind turbines. This research presents a generic framework for the maintenance planning of wind turbines. Within the proposed framework, the performance degradation of wind turbines over time is characterized with stochastic damage growth models. Costs incurred during the life span of wind turbines are modeled mathematically to compute the accumulated risks involved in the O&M process. Probabilistic models are used to characterize the maintenance activities and a unified maintenance optimization platform is then formulated to derive optimal maintenance strategies. A case study of offshore wind turbines is used to demonstrate the proposed methodology.