Wind farms are becoming a leading source for renewable energy. The increased reliance on wind energy makes wind farm control systems attractive targets for attackers. This talk explains how wind farm control networks work and how they can be attacked in order to negatively influence wind farm operations (e.g., wind turbine hijacking). Specifically, implementations of the IEC 61400-25 family of communications protocols are investigated (i.e., OPC XML-DA). This research is based on an empirical study of a variety of U.S. based wind farms conducted over a two year period. We explain how these security assessments reveal that wind farm vendor design and implementation flaws have left wind turbine programmable automation controllers and OPC servers vulnerable to attack. Additionally, proof-of-concept attack tools are developed in order to exploit wind farm control network design and implementation vulnerabilities.