The initiation of a government environmental criminal investigation or discovery of a serious compliance issue often triggers the need to conduct an internal investigation. The decision to conduct an internal investigation entails complex issues regarding the scope of the investigation, who conducts the internal investigation, how to conduct the investigation in parallel with an ongoing government investigation, how to use experts to assist in the investigation, how to manage whistleblowers, when and how to disclose the results of the investigation to regulators and prosecutors, and many other issues. This Article addresses internal corporate investigations in the environmental compliance context and provides practical tips for handling the investigations.