The Permit-class submarine (known as the Thresher class until the lead boat USS Thresher was lost) was a class of nuclear-powered fast attack submarines (hull classification symbol SSN) in service with the United States Navy from the early 1960s until 1996. They were a significant improvement on the Skipjack class, with greatly improved sonar, diving depth, and silencing. They were the forerunners of all subsequent US Navy SSN designs. They served from the 1960s through to the early 1990s, where they were decommissioned due to age. They were followed by the Sturgeon and Los Angeles classes.