A tool for milling a window in a casing (3) of a borehole, the tool comprising: a main body having a longitudinal axis wherein, when the tool is run into a borehole, the longitudinal axis is substantially parallel with the longitudinal axis of the borehole, the main body comprising a deflection surface (7) which is disposed at an angle with respect to the longitudinal axis; a milling arrangement comprising a milling head (11) and a guidance surface (13), wherein the guidance surface is disposed at an angle with respect to the longitudinal axis of the tool and is in contact with the deflection surface; and a drive arrangement (14) which is operable to effect relative longitudinal movement between the main body and the milling arrangement.