A Starfleet programme under which officers serve temporarily aboard the vessels of other services, which sent Riker to a Klingon ship and brought Mendon and later Kurn to the Enterprise.
The Officer Exchange Program is intended to build understanding between Starfleet and other fleets by letting officers do each other's jobs for a time.
It is the subject of A Matter of Honor. Riker volunteers to become the first Federation officer to serve on a Klingon vessel and joins the IKS Pagh as first officer under Captain Kargan, having studied Klingon customs and food with some relish. In the same exchange the Benzite ensign Mendon comes aboard the Enterprise, where his habit of withholding a problem until he has solved it delays the discovery of a hull-eating organism on both ships. Kargan assumes a Federation attack, and Riker has to take command of the Pagh to avert a battle.
The programme works in the other direction in Sins of the Father, when the Klingon commander Kurn serves as first officer of the Enterprise.