Constitution-class starships were the premiere front-line Starfleet vessels in the latter half of the 23rd century. They were designed for long duration missions, with minimal outside support. They were best known for their celebrated missions of galactic exploration and diplomacy which typically lasted up to five years.
The first Constitution-class starships were launched in 2245, and served as Starfleet's leading ships for the rest of the century. The Constitution-class's also served as the mighty deterrent to both the Klingon and Romulan Empires, numerous times taking part in conflicts which determined the fate of the entire Federation, if not the Alpha Quadrant itself.
In the early 2270s, the Constitution-class starships underwent a major refit program. The actual refitting took eighteen months of work, and essentially a new vessel was built onto the bones of the old. Virtually every system was remodeled and redesigned, thus the Constitution-class starships continued in service for a further twenty years.