The Battle of Vlaardingen was fought on 29 July 1018. The German emperor sent an army towards western Frisia to subdue the rebellious Count Dirk III. However, the imperial army was defeated by the Vlaardingers and fled in panic. Our knowledge of the battle is based on three chronicles, written shortly after the date: De diversitate temporum by the monk Alpertus of Metz, the Chronicon of Thietmar, bishop of Merseburg, and in the Cambrai Bishop’s Chronicle. Also, recent archaeological discoveries shed some light on Vlaardingen in the 11th century.