A tool for milling a window in the casing 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 which is disposed at an angle with respect to the longitudinal axis; a milling arrangement comprising a milling head and a guidance portion, the guidance portion having a body which includes, on one side thereof, a guidance surface, 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 which is operable to effect relative longitudinal movement between the main body and the milling arrangement, wherein the drive arrangement comprises one or more actuating members extending between the main body and the milling arrangement, and which are operable to effect relative longitudinal movement between the main body and the milling arrangement, and wherein the or each actuating member is connected to the milling arrangement, or to the main body, in such a way that the actuating member may slide laterally with respect to the milling arrangement or main body, and wherein, when the drive arrangement is operated to effect relative longitudinal movement between the main body and the milling arrangement, leading to lateral motion of the milling arrangement with respect to the main body, the rotational orientation of the guidance surface of the milling arrangement with respect to the main body remains substantially unaltered. 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 which is disposed at an angle with respect to the longitudinal axis; a milling arrangement comprising a milling head and a guidance portion, the guidance portion having a body which includes, on one side thereof, a guidance surface, 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 which is operable to effect relative longitudinal movement between the main body and the milling arrangement, wherein the drive arrangement comprises one or more actuating members extending between the main body and the milling arrangement, and which are operable to effect relative longitudinal movement between the main body and the milling arrangement, and wherein the or each actuating member is connected to the milling arrangement, or to the main body, in such a way that the actuating member may slide laterally with respect to the milling arrangement or main body, and wherein, when the drive arrangement is operated to effect relative longitudinal movement between the main body and the milling arrangement, leading to lateral motion of the milling arrangement with respect to the main body, the rotational orientation of the guidance surface of the milling arrangement with respect to the main body remains substantially unaltered.