This PhD project is focused on the investigation of the power performance of a wind turbine which is part of a wind farm. The purpose is to determine how the surrounding turbines affect the individual turbine performance. Most of the attention is paid on wake and global blockage effects as well as on how these effects are related to characteristics of the inflow such as wind shear and turbulence. Additionally, this investigation is conducted by both performing simulations and using measurements from nacelle lidars. Therefore, there is also the aim to establish and test a methodology to measure the previously mentioned characteristics with a nacelle-mounted lidar.