Offshore wind energy has been growing rapidly due to its capacity for utilizing much larger turbines and thus higher power generation compared to onshore. With the increasing size of offshore wind turbine rotors, the design criteria used for the blades may also evolve. Increased flexibility in blades causes them to be more susceptible to experiencing flow-induced instability. One of the destructive aero-elastic instabilities that can occur in flexible structures subjected to aerodynamic loading is coupled-mode flutter. Coupled-mode flutter instability has not been a design driver in the current wind turbine blades, however, considering the industry tendency in utilizing longer and lighter blades, it needs very closeattention. Long-span bridges, aircrafts and turbomachines are the most common engineering devices subject to flutter. In recent years, a few studies have focused on flutter instability in wind turbine blades. Coupled-mode flutter in wind turbine blades is the result of the interaction between a torsional mode and a flapwise mode. The two structural modes coalesce at a critical flow velocity and result in a negative damping that cannot be compensated by structural damping. Contrary to the stall flutter which is the result of separation and reattachment of the flow due to high angles of attack, classical flutter occurs in the attached flow regime and may occur in pitch-regulated wind turbines. The aim of this thesis is to provide a thorough study of the coupled-mode flutter in wind turbine blades. For this purpose coupled-mode flutter is studied both through theoretical modeling and wind tunnel experimentations. Parametric studies are performed on three MW-size wind turbine blades and it is shown that the ratio between the torsional and flapwise natural frequencies, as well as the magnitude of the 1st torsional natural frequency significantly influence the onset of flutter. To investigate the influence of uncertainty in system’s parameters on the onset of flutter, Monte Carlo simulations are conducted assuming randomness in both flow forces and structural properties. It is shown that the safety margin between the flutter onset and the rated rotor speed shrinks and in some cases vanishes when the randomness is considered. Different reliability methods are used to mitigate the Monte Carlo simulations and a new reliability method which is developed, is proven to be a viable substitute for the Monte Carlo with much less computing time. Coupled mode flutter of fixed and rotating highly flexible airfoils is also studied and the influence of static deflection on the flutter characteristic is shown and validated through conducting experiments in a wind tunnel. A small scale wind turbine is designed to study the aero-elastic instabilities in rotating blades. A set of experiments is carried out in a wind tunnel and the failure of the small scale blades due to the aero-elastic instability is captured.