The Posse Comitatus Act was created to limit the military's role in civilian law enforcement. The original act was passed in June 1878 as part of the Army appropriation bill. The National Security Act of 1947 further directed the Secretary of Defense to publish regulations that do not permit direct participation by the Army or Air Forces in search, seizure, arrest, or other similar activities, except when permitted by law. The Posse Comitatus Act does not apply the National Guard formation while under state control and therefore allows governors the flexibility to utilize the Guard in support of law enforcement missions. This exception is lost once National Guard formations are called to serve on Active Duty. This book will explore the history of the use of a Posse Comitatus before and during the Revolutionary War, the Civil War, and present day situations. The book will examine when the Posse Comitatus Act applies and when it does not apply to active duty and National Guard formations. Exceptions to the law which have been granted by Congress for counter drug operations, suppression of insurrection and emergency situations related to weapons of mass destruction will also be examined. Three situations will be examined in detail including the Los Angeles Riots in 1992, the World Trade Organization Ministerial Conference in Seattle in 1999, and the use of Military Police in Samson, Alabama in 2009. Finally, this book will examine command and control issues related to the use of Federal Armed forces in support of local law enforcement. The Posse Comitatus Act restricts the Federal Government in all but extraordinary situations. Its intent to limit federal involvement in domestic affairs is still sound. In the future, commanders at all levels should be required to receive training on the Posse Comitatus Act to ensure understanding and intent. Congress should consider allowing active duty units to serve under state command and control and the Posse Comitatus Act should be left as a federal law.