'Winter Line' was the Allied semi-official designation of the series of German defence lines in Italy, in the area to the south of Rome, constructed primarily by the Organisation 'Todt' (1943/44).

The primary defensive position was the 'Gustav-Linie' extending across Italy from a point just north of the outflow of the Garigliano river into the Tyrrhenian Sea in the west, through the Apennine mountains to the mouth of the Sangro river on the coast of the Adriatic Sea in the east. The centre of the line, where it crossed the main route north to Rome (Highway 6) which followed the Liri river valley, was anchored around the mountains behind the town of Cassino including Monte Cassino, on which was situated an old abbey that dominated the entrance to the Liri river valley (a main route to Rome), and Monte Cairo which gave the defenders clear observation of potential attackers advancing towards the mouth of the Liri river valley.

On the western side of the Apennines there were two subsidiary lines: the 'Bernhardt-Linie' in front of the main 'Gustav-Linie' positions and the 'Hitler-Linie' some 5 miles (8 km) to the rear of the 'Gustav-Linie'.

The 'Winter Line' comprised a series of interlinked positions fortified with gun emplacements, concrete bunkers, turreted machine gun emplacements, barbed wire and minefields, and was the strongest of the German defensive lines south of Rome. About 15 German divisions were employed in the defence, and it took the Allies from mid-November 1943 to late May 1944 to fight through all the various elements of the Winter Line in battles which included those at Monte Cassino and Anzio.

Some authorities define the 'Bernhardt-Linie' as crossing Italy from coast to coast following not just the western defensive positions described above but incorporating also the eastern defences of the 'Gustav-Linie'. Other authorities use the designation Winter Line interchangeably with the 'Gustav-Linie'.