Abstract: A coupling serves for leakproof connection of two shaft-like medical instruments (10, 90) along their shaft axis (53, 97), in particular for connection of an arthroscope shaft to an optical system. A coupling element (80) is provided, arranged on a first instrument (10) and having a contact surface (66), against whose contact surface (66) a contact surface (106) of a coupling element (92) of the second instrument (90) can be applied. Also provided is an interlock system for releasable interlocking of the two coupling elements (80, 92), the contact surfaces (64, 106) resting against one another in leakproof fashion.