Traditionally, wind turbine controllers are designed using first principles or linearized or identified models. The aim of this paper is to show that with an automated, online, and model-free tuning strategy, wind turbine control performance can be significantly increased. For this purpose, iterative feedback tuning (IFT) is applied to two different turbine controllers: drivetrain damping and collective pitch control. The results, obtained by high-fidelity simulations using the NREL 5MW wind turbine, indicate significant performance improvements over baseline controllers, which were designed using classical loop-shaping techniques. It is concluded that iterative feedback tuning of turbine controllers has the potential to become a valuable tool to improve wind turbine performance.