Situated at the Pilsudski Square.
It is a monument to the nameless heroes fallen in battle for Poland's freedom - in World War I and the subsequent Polish-Soviet War and also the ashes and soil brought from WWII battle places.

It is one of Poland' most important national symbols. In remembrance official visiting delegations place their wreaths on this spot to pay homage to the soldiers lost in the wars.
The guards are changed at the Tomb of the Unknown Soldier every hour.

The Tomb was designed by the famous Polish sculptor, Stanislaw Kazimierz Ostrowski.
It was located in 1925 within the arcade that linked the two symmetric wings of the Saxon Palace, which was completely destroyed by the Wehrmacht after the Warsaw Uprising in December 1944.
After the war only the small part of the palace containing the Tomb itself was restored, but soon it will be rebuilt in original shape.

More info: http://en.wikipedia.org/wiki/Tomb_of_the_Unknown_Soldier,_Warsaw