Wind turbines (WTs) are typically designed for a 20-year life (see Chapter 16). However, the design life is a theoretical parameter and is also called the planned service life. The load-bearing capacity of the structure and suitability for use must be demonstrated for this period. At the end of the 20-year period, there is generally an interest in continuing to operate the WT beyond this period. From the other hand, extended life of wine turbines also lead to higher O&M costs and a greater risk of costly structural failures and increased safety risks.