Wind turbine power output has grown massively over the past few decades, and this has been achieved in part by increasing the size of rotors. But the size of rotors is now limited by structural constraints as well as space constraints in wind farms. It is therefore important to use other innovative methods to increase wind turbine capacity without increasing size. One way to achieve this is by the use of winglets. Winglets increase power output by reducing tip effects, thereby producing a more efficient distribution of forces over the blade. The art of designing winglets is to find the best trade-off between the increase in profile drag of the winglet itself and the reduction of induced drag that the winglet provides. To do this, it is very important to fully understand the aerodynamics of winglets on wind turbine blades. High-fidelity methods like CFD are capable of producing accurate and detailed flow fields and are able to offer greater insight into the complex aerodynamics of winglets on rotors. However, this comes at great computational cost which might be infeasible in the design and optimization of winglets. More common and cheaper models like the BEM method are incapable of modelling winglets and other out-of-plane features. The Lifting Line Method is a middle ground that is capable of simulating winglets but is also comparatively inexpensive. The goal of this thesis is to study the performance of the Lifting Line method, in particular, ECN Aeromodule's AWSM Free-Wake Vortex Lifting Line code in simulating the case of winglets mounted on wind turbines. AWSM results are compared with results of normal and tangential forces and circulation distribution from a validated OpenFOAM model. The results show that over the outboard section of the blade and over the span of the winglet, AWSM performs well in predicting the performance of the blade-winglet configuration. This study shows that AWSM is a reliable tool for the design and optimization of winglets on wind turbine blades at a much lower cost than higher fidelity methods like CFD.