A winch assembly (10) comprises a winch drum (18) rotatable about a first axis and carrying a spoolable medium, such as wireline (14) for use in deploying and retrieving tooling into and from a well bore. A carriage sheave (30) mounted on a drive screw (34) is positioned adjacent the winch drum (18), wherein the drive screw (34) is rotatable about a second axis substantially parallel to the first axis. In use, the carriage sheave (30) engages the spoolable medium and is translated by the drive screw (34) in reverse directions to follow the fleeting motion of the spoolable medium as the medium is paid out and/or in from the winch drum (18). In a disclosed embodiment, the winch drum (18) is mounted vertically on a tubular body (22), through which tubular body (22) the spoolable medium extends into the well bore. In one embodiment the winch assembly (10) forms part of a subsea well intervention system.