Fatigue life prediction of wind turbine rotor blades is a very challenging task, as their failure is led by different failure types that act synergistically. Inherent defects like wrinkles, fiber misalignments, and voids that can be introduced during fabrication can constitute potential damage initiation points and rapidly develop to failure mechanisms like matrix cracking, transverse-ply cracking, interface cracking, debonding, fiber breakage, etc. Different methods have been established to address this problem; others based on phenomenological and other on actual damage mechanics modeling. This chapter aims to provide an overview of fatigue life modeling and prediction methodologies for composite materials and structural composite elements comprising a wind turbine rotor blade under complex loading conditions.