The suppression of piracy and other forms of maritime violence was a keystone in the colonisation of Southeast Asia. Focusing on what was seen in the nineteenth century as the three most pirate-infested areas in the region - the Sulu Sea, the Strait of Malacca and Indochina - this comparative study in colonial history explores how piracy was defined, contested and used to resist or justify colonial expansion, particularly during the most intense phase of imperial expansion in Southeast Asia from c.1850 to c.1920. In doing so, it demonstrates that piratical activity continued to occur in many parts of Southeast Asia well beyond the mid-nineteenth century, when most existing studies of piracy in the region end their period of investigation. It also points to the changes over time in how piracy was conceptualised and dealt with by each of the major colonial powers in the region - Britain, France, the Netherlands, Spain and the United States.