This paper presents results of a study to investigate the effect of leading edge erosion on the aerodynamic performance of a wind turbine airfoil. The tests were conducted on the DU 96-W-180 wind turbine airfoil at three Reynolds numbers between 1 million and 1.85 million, and angles of attack spanning the nominal low drag range of the airfoil. The airfoil was tested with simulated leading edge erosion by varying both the type and severity of the erosion to investigate the loss in performance due to an eroded leading edge. Tests were also run with simulated bugs on the airfoil to assess the impact of insect accretion on airfoil performance. The objective was to develop a baseline understanding of the aerodynamic effects of varying levels of leading edge erosion and to quantify their relative impact on airfoil performance. Results show that leading edge erosion can produce substantial airfoil performance degradation, yielding a large increase in drag coupled with a significant loss in lift near the upper corner of the drag polar, which is key to maximizing wind turbine energy production. Copyright © 2013 John Wiley & Sons, Ltd.