In this work the aerodynamic performance of winglets of modern sailplanes were investigated. First isolated wings with winglets were designed and investigated and compared to the wings without winglets, showing the fundamental working principle and effectiveness of winglets. Then a complete sailplane was designed with an integrated inviscid flow method with XFLR5  combined with Navier-Stokes CFD with Siemens PLM STAR CCM+ computations. In such a way a modern sailplane was designed for operation with winglets. Using this modern sailplane design the impact of winglets on sailplanes was investigated in detail. This modern sailplane was simulated with CFD in the configurations without winglets and with two modern advanced winglets and their aerodynamic characteristics and performance were investigated and compared. All aerodynamic results are analyzed and shown in detail.