Abstract: Control apparatus for a sub-sea wellhead installation, for example of a satellite well, is disclosed which includes a plurality of control devices 80 each for providing a control function for the installation 10 and a connector 40, 45, 50 for connecting the devices 80 to the installation 10, each control device being independently engageable and disengageable with the connector 40, 45, 50. The connector includes a manifold 40 to which are connected a plurality of receptacles 50, each control device 80 being engageable with the the receptacle 50. The receptacles and control devices are preferably provided with complementary engageable portions 52, 54, 56; 82, 84, 86, which include hydraulic and electrical connectors 58, 60, 62, 70, 72; 88, 90, 92, 98, 100, so that control signals may be passed via the connectors 40, 45, 50 to the control devices 80 from a command installation and actuation signals may be passed from the control devices 80 through the connector 40, 45, 50 to devices on the installation 10.