The Munroe effect refers to the partial focussing of blast energy caused by a hollow or void cut into a piece of explosive, a property which is exploited by a shaped charge.

Explosive energy is released directly away from (normal to) the surface of an explosive, so shaping the explosive will concentrate the explosive energy in the void. If the void is properly shaped (usually conically), a high-velocity jet of plasma will form.

It is named after Charles E. Munroe, who discovered it in 1888. Whilst working at the Naval Torpedo Station at Newport in the United States, he noticed that when a block of guncotton with the manufacturer's name stamped into it was detonated next to a metal plate, then the lettering was cut into the plate. If letters were raised in relief above the rest of the guncotton then the letters on the plate would also be raised above its surface. In 1910, Egon Neumann of Germany discovered that TNT containing a conical indentation would cut through a metal plate which would normally only be dented by that quantity of explosive. However, the military usefulness of this effect was not appreciated until the Second World War, the first application possibly being the British No. 68 rifle grenade which entered service in May 1940.

In modern military applications, a Munroe-effect shaped-charge warhead can be expected to penetrate solid steel armor equal to 150-250% of the warhead diameter, though it will tend to be somewhat less effective against modern composite armors and reactive armor which was developed specifically as a counter to shaped charge weapons.

In peaceful engineering applications, shaped charges are very useful for cutting steel girders, perhaps in order to demolish an old building.

Compare to the Misznay-Schardin effect.