A System Monitor is a security program tasked with maintaining system integrity from internal threats such as gridbugs, rogue programs and viruses. Flynn copied Tron, a security program from the ENCOM mainframe, to the TRON system to act as head of system security and maintain order. After the emergence of the ISOs, Clu and several system monitors performed terrorist attacks against ISO-populated areas of the Grid. Despite this, most system monitors continued to perform their role of protecting the TRON system. When the Abraxas virus was unleashed, the system monitors were quickly repurposed into sentries and sent to fight against infected programs.