This paper presents a comprehensive study on the model predictive control (MPC) of doubly fed induction generators (DFIG) in wind energy conversion systems (WECS); in particular the MPC of the rotor side converter. The general principle of prevalent MPC strategies is discussed to introduce the theoretical framework in the first place. Furthermore, mainstream and high-performance MPC methods of DFIGs have been identified as model predictive current control (MPCC), model predictive torque control (MPTC), and model predictive power control (MPPC). Starting from analyzing dynamic models, these MPC strategies are investigated in terms of operating principles and technical developments. The paper further investigates implementation of MPC strategies in unbalanced grids. Based on the quantitative and qualitative analysis of several case studies, performance of various MPC schemes is compared and their special features are identified. Guidelines for designing MPC strategies of DFIGs in WECS are provided.