An experimental study was conducted to investigate the performance of wind turbines sited over hilly terrains in order to elucidate the underlying physics to explore/optimize design of wind turbines sitting over complex terrains for higher power yield and better durability. The experiments were conducted in an atmospheric boundary layer wind tunnel with wind turbines sited over a two-dimensional hill. In addition to measuring dynamic wind loads and the power outputs of the wind turbines, detailed flow field measurements were also made to quantify the flow characteristics of the surface wind and the wake interference among multiple wind turbines over hilly terrain. The detailed flow field measurements were correlated with the wind load and power output measurements of the wind turbine models to explicate the physical mechanisms associated with power generation and fatigue loads acting on the wind turbines for the optimal design of the wind turbine layout over complex terrain.