The Judge Advocate General (often shortened to JAG) is the senior officer within Starfleet charged with overseeing the legal matters of it's members. This officer's department is known as the Judge Advocate General's Office.
In 2355, Captain Phillipa Louvois presided over the investigation into the loss of the USS Stargazer. In 2365, she was the commander of the JAG office of Sector 23. One of Captain Louvois' first rulings was that Lieutenant Commander Data was not the property of Starfleet. (TNG: "The Measure of a Man")
The Judge Advocate General's office was responsible for the investigation into the mutiny that occured aboard the USS Pegasus in 2358. (TNG: "The Pegasus")
Rear Admiral Bennett was the JAG in 2373 who ruled on Richard Bashir's criminal actions when he pled guilty to the resequencing of his son's DNA. (DS9: "Doctor Bashir, I Presume?")
Additional Members
Additional references
Template:Branches of Starfleet