High towers are flexible and therefore and in the case of very large rotors, the flexibility of the tower is moderate compared with the blades. Despite this moderate flexibility, tower vibrations have an impact on the rest of the structure. In the modelling of the tower, three increasing levels of complexity can be observed in the literature. One is the tower with a large mass at the top without a rotor. The dynamics consider only the first natural frequency in the fore–aft direction. The second motion in a sideway direction with similar dynamics can also be added. The second level consists of models that reflect the blade–tower interaction but only in the flapwise/fore–aft direction for the first natural frequencies of the blade and tower. The third level is determined by the number of external forces and moments considered in the model. The models are presented in this order in the chapter.