An experimental study focusing on the change of the aerodynamic performance of a wind turbine with the employment of trailing-edge serrations is presented. The design procedure which starts from the serration design, together with the experimental wind tunnel testing and the installation in an already operating wind turbine is discussed. A prediction methodology to estimate the aerodynamic performance change of the machine with the blade add-on is validated by in-field measurements. Wind tunnel experiments for the validation of the original serration design have been carried out on a Nordex AD030 airfoil with a relative thickness of 30% and on a NACA64(3)418 airfoil with 18% thickness. The two airfoils are respectively used in this study due to their peculiar characteristics. While the first has been designed for relatively high Cl/Cd performance versus structural integrity (i.e. relatively high thickness), the second one is a typical reference airfoil used in wind turbines for its interesting Cl/Cd performance in both laminar and rough conditions. Clean and rough conditions have been tested in order to prepare a database for the analysis of the full turbine in several wind conditions. Aerodynamic forces with the serrated trailing-edge extensions are measured for different angles of attack and serration flap angles. The results are further discussed and employed for the analysis of the full rotor. Results already show that an increase in the flap angle is typically associated with an increment in lift, but not necessarily in drag. This has a beneficial effect on the operational regime of the machine when taken into account. The influence of the trailing-edge serrations on an operating wind turbine has been quantified in terms of total loads and energy production. The power curves with and without the trailing-edge installations are further analyzed and compared with the theoretical predictions. (C) 2019 Elsevier Ltd. All rights reserved.