Aviation fuel is critical to aviation safety, and the potential for contamination of that fuel is a hazard that the industry must mitigate. This resource aims to inform the aviation industry of three aircraft fuel contamination events that occurred in November 2017 at Omaha, Nebraska, August 2018 at Opa Locka, Florida and May 2019 at Punta Gorda, Florida, where diesel exhaust fluid (DEF) was identified as the contaminant. It details how those events transpired, offers a safety analysis of those events and the risk that DEF contamination poses to safety, and proposes recommendations for aircraft operators, fixed base operators (FBOs) and fuel suppliers to adopt to reduce the likelihood of another inadvertent DEF fuel contamination event.