The 'Hollandsche Waterlijn' was a Dutch line of defensive positions based on water lines (1940).

The first 'Hollandsche Waterlijn' was a series of water-based defences conceived by Prince Maurits van Orangje early in the 17^th century and realised by his half brother, Frederik Hendrik. Combined with natural bodies of water, this water line could be used to transform Holland into what was in effect an island. In the 20^th century, the line was extended to include Utrecht.

Work on the line began in 1629. Sluices were built in dikes and fortresses and fortified towns were created at strategic points along the line with guns sited to cover the dikes which traversed the water line. The water level in the flooded areas was carefully maintained at a level deep enough to make an advance on foot precarious and shallow enough to rule out effective use of all boats other than the flat-bottomed gun barges used by the Dutch defenders. Under the water level additional obstacles like ditches and staked pits were hidden. The trees lining the dikes, which constituted the only roads through the line, could be turned into abatis in time of war. In winter the water level could be manipulated to weaken ice covering, while the ice itself could be used when broken up, to form further obstacles that would expose advancing troops longer to fire from the defenders.

After the final defeat of the French Emperor Napoleon in 1815, the United Kingdom of the Netherlands was established, and soon after this King (Willem Frederik I decided to modernise the water line and also relocate part of it to the area lying to the east of Utrecht. This created the new 'Hollandsche Waterlijn', which was farther extended and modernised in the 19^th century with fortresses containing round gun towers. The line was mobilised but not attacked during the Franco-Prussian War (1870/71) and World War I (1914/18).

By 1939 and the start of World War II, most of the earth and brick fortifications were appreciated as being too vulnerable to modern artillery and bombs to withstand a protracted siege, and in an effort to remedy this limitation many concrete pillboxes were added. However, the Dutch had decided to use a more eastern main defence line, the 'Grebbelijn' (Grebbe line), and the water line became a secondary feature of Dutch defensive planning. When the German broke though the 'Grebbelijn' on 13 May, the Dutch field army was withdrawn to the 'Hollandsche Waterlijn'. However, modern tactical thinking had devised the methods which could circumvent fixed defence lines. While the Dutch army was fighting along the 'Grebbelijn', small German airborne forces captured the southern approaches into the heart of 'Vesting Holland' by surprise descents at key points such as the bridges at Moerdijk, Dordrecht and Rotterdam. When the Dutch resistance did not end, the Germans forced the Dutch into surrender by bombing Rotterdam, and threatening the same for Utrecht and Amsterdam, so in the course of the Battle of the Netherlands in May 1940 there was no fighting along the line itself.

In its definitive form, the 'Hollandsche Waterlijn' was 53 miles (85 km) long and possessed a width varying between 1.85 and 3.1 miles (3 and 5 km), and included 60 defensive works and 10 basins in its flood zone.