In recent years, the research on Vertical Axis Wind Turbines (VAWTs) have been paving the way for the very large-scale (10-20 MW) floating offshore applications. For a cost effective utilization, VAWTs need to deliver superior aerodynamic and structural performance. Nowadays, the complex ow phenomenon of VAWTs have been better understood and the computational tools have reached to a mature level. It is time to explore the capabilities of this concept and improve its aerodynamic features by innovative design ideas. The main goal of this thesis is to enhance the performance of the lift-driven VAWTs by customized airfoil design and the active ow control with trailing edge aps. To achieve the research goals, four main lines of work are carried out: 1) performance exploration of an actuator cylinder, 2) design of the azimuthal flap control sequences, 3) comparison between the Actuator Cylinder Model and Panel Model in the presence of the active flap control and 4) airfoil design that is specified for effective flap operation on a VAWT blade. The flap control sequences are optimized with the use of the Actuator Cylinder Model for three aerodynamic objectives. These objectives are aiming to improve the power efficiency by maximizing the CP , provide a rated power control by minimizing the CP and decrease the cyclic load ranges by minimizing the CT . Whereas, RFOIL and NSGA-II genetic algorithm are coupled for the airfoil design, which aims for objectives such as superior single airfoil performance, extended flap sensitivity and high flap-wise bending stiffness. For a rotor solidity of 0.1 operating at tip speed ratio of 4, a 10% active flap is able to increase the CP by 7% , decrease the CP by 10% and alleviate 12% of the CT by sacrificing 3% of the CP . It is shown that depending on the solidity, tip speed ratio and the flap authority these figures could be increased. It is possible to brake the rotor with a relatively larger flap authority. The airfoils designed in this work are slightly cambered and span from 27% to 35% thickness, deliver higher maximum CP than a NACA 0018, show good performance in the dirty conditions and have high flap effectivity. Overall, the performance gains acquired by the new airfoils and flap control promises extensive improvements in multiple features of VAWTs such as power effciency, power control, load control that lead to reduced weight and decreased Cost of Energy.