Wind turbines are designed to operate optimally at a certain set of ambient conditions, termed the design point. An unavoidable consequence of wind turbine design is that away from this point, eciency can drop drastically which inhibits energy conversion, especially in highly varying winds. Numerous strategies have been employed, for example blade pitch and torque control systems, which mitigate these losses, but such systems are economically costly and are usually only present in larger scale applications. For small Horizontal-Axis Wind Turbines (HAWTs) as well as most Vertical Axis Wind Turbines (VAWTs), pitch control strategies in particular are either too complex or simply not economically feasible. Recent research into flexible bladed wind turbine rotor design has shown that continuous, passive blade morphing can increase aerodynamic lift, reduce drag, and even delay stall for two-dimensional airfoil sections. These studies, however, do not take into account centrifugal or gravitational loadings which can change the morphing direction of the flexible material. In this thesis, a fully three-dimensional Fluid-Structure Interaction (FSI) solver is developed and used to simulate a flexible HAWT rotor, with comparisons to experimental results, also conducted herein. The experimental findings, which compare geometrically identical rigid and flexible rotors, show a marked increase in average torque and operational envelope over a wide range of flow conditions. Through the analysis of the associated FSI simulations, these performance increases are attributed to small changes in local attack angles, acting to mitigate losses associated with flow separation. After validation with experimental data, the FSI solver is then utilized to investigate the eects of flexible blade design on a VAWT, the first analysis of its kind. Results indicate that eciency increases are realized not by improving the maximum rotor torque, which varies along VAWT rotation, but by increasing the torque minima through passive deflection of the blades. This morphing action augments the local attack angle to decrease the magnitude of low-pressure regions created due to blade stall, acting to improve average rotor torque and increase energy capture of the flexible VAWT design when compared to a geometrically identical rigid one.