Abstract: A versatile apparatus (2) for controlling the transmission of rotational forces between first and second rotatable members preferably takes the form of a clutch that can be activated by axially shifting an internal actuation member (140) through the introduction of either liquid or gaseous mediums. The clutch includes first and second transmission members (177, 281) which are rotatably mounted within a housing (5) and adapted to be attached to the first and second rotatable members respectively. A coupling assembly (226), preferably including first and second sets of axially spaced interface elements (252-254 and 274, 275), is interposed between the transmission members (177, 284) within the housing and axial movement of the actuation member (140) causes the interface elements (252-254 and 274, 275) to be frictionally engaged.