Abstract: A releasable coupling especially for quick inter-changeable tool mounting purposes comprises a socket member, e.g. for mounting in a machine tool drive spindle and a spigot shown of taper form and received by a corresponding socket in the socket member. The socket member has a driving engagement with the spigot and a shank of the latter may consist of or carry any one of a variety of tooling. A collar movable between locking and release positions about the socket member has internal cam formation acting on retaining elements such as balls in bores through the socket member which balls engage an annular groove about the spigot to retain it in the socket. Release movement of the collar enables the balls to radially retract for withdrawal of the spigot.