Lasne is the richest municipality in Wallonia, measured by the average taxable income of the inhabitants.

The municipality consists of the following districts: Couture-Saint-Germain, Lasne-Chapelle-Saint-Lambert, Maransart, Ohain, and Plancenoit (including the hamlet of Marache).

Part of the Battle of Waterloo was fought in the village of Plancenoit, notably around the church and the fortified farm of La Papelotte.

The current historic area commemorating the battle is jointly administered by the municipalities of Lasne, Waterloo and Braine-l'Alleud.

The name Lasne comes from the Celtic word meaning flat or calm, presumably referring to the slow moving river.