Abstract: A whipstock assembly (12) for use in deviating a wellbore (10) of a well is disclosed, the whipstock assembly comprising a whipstock (18) having an inclined deflection surface (20); a milling assembly (14) comprising a mill (16) for milling a window (49) in a wall (48) of the wellbore and forming a rathole (50) extending from the wellbore; a releasable joint (22; 122) by which the milling assembly is releasably connected to the whipstock, so that the milling assembly can be separated from the whipstock within the wellbore and subsequently deflected towards the wellbore wall by the deflection surface of the whipstock; at least one sensor (188) provided in the milling assembly, for measuring at least one parameter of the rathole, the at least one parameter relating to a trajectory of the rathole; and a communication device (190) associated with the at least one sensor, for communicating data relating to the at least one parameter to an operator of the milling assembly.