A running tool assembly for installing a tubing or casing hanger in a wellhead housing, comprising: a first body configured to support the tubing or casing hanger; a piston chamber formed in the running tool assembly; an actuation piston axially moveable between a first position and a second position axially spaced from the first position; wherein, the actuation piston is coupled to an axially extending sleeve and when the running tool assembly is coupled to the tubing or casing hanger and the hanger is positioned in the wellhead housing, the sleeve extends to locking means (420,422) disposed about the tubing or casing hanger and the piston is configured to actuate from the first position to the second position in response to a pressurization of the piston chamber and actuate the sleeve 340 to operate the locking means (420,422) disposed about the tubing or casing hanger and thereby lock the tubing or casing hanger to the wellhead housing; and wherein the running tool assembly comprises a central passage having a minimum diameter that is equal to or larger in size than a minimum diameter of a central passage of the tubing or casing hanger; characterised in that the running tool assembly comprises a second body releasably coupled to the first body and axially displaceable relative to the first body; and the running tool assembly further comprises a control line sub with a locking member to prevent or permit axial movement of the control line sub relative to the second body, and the running tool assembly is configured such that axial displacement of the second body relative to the first body enables movement of the locking member of the control line sub to an unlocked position permitting axial movement of the second body relative to the control line sub. a first body configured to support the tubing or casing hanger; a piston chamber formed in the running tool assembly; an actuation piston axially moveable between a first position and a second position axially spaced from the first position; wherein, the actuation piston is coupled to an axially extending sleeve and when the running tool assembly is coupled to the tubing or casing hanger and the hanger is positioned in the wellhead housing, the sleeve extends to locking means (420,422) disposed about the tubing or casing hanger and the piston is configured to actuate from the first position to the second position in response to a pressurization of the piston chamber and actuate the sleeve 340 to operate the locking means (420,422) disposed about the tubing or casing hanger and thereby lock the tubing or casing hanger to the wellhead housing; and wherein the running tool assembly comprises a central passage having a minimum diameter that is equal to or larger in size than a minimum diameter of a central passage of the tubing or casing hanger; characterised in that the running tool assembly comprises a second body releasably coupled to the first body and axially displaceable relative to the first body; and the running tool assembly further comprises a control line sub with a locking member to prevent or permit axial movement of the control line sub relative to the second body, and the running tool assembly is configured such that axial displacement of the second body relative to the first body enables movement of the locking member of the control line sub to an unlocked position permitting axial movement of the second body relative to the control line sub.