In 2013, the FBI started posting a new most wanted list—not for kidnappers, murderers, or armed robbers, but for online crooks. These "Cyber''s Most Wanted" criminals have committed serious offenses ranging from hijacking Internet traffic to spying on businesses and governments. They steal online passwords and financial information, break into online bank accounts, install malicious software on computers, and hack into computers via spam and phishing e-mails. Cyber crime is serious business. Internet security companies report that worldwide cyber crimes cost consumers about $113 billion per year. Cyber crimes cost businesses around the world even more, up to $500 billion each year. Technology experts continue to improve security software, but the attacks keep coming. Hackers are hitting electrical power grids, oil and gas pipelines, and other critical national infrastructures. Cyber criminals relentlessly strike corporate computers, trying to steal financial data, industrial blueprints, and business plans. They steal passwords from personal computers, which in turn can give them access to credit card, Social Security, and bank account numbers. Some cyber criminals steal military secrets. Others disrupt the workings of governments and political organizations. Many cyber crooks are lone individuals while others work in gangs. Many cyber criminals use botnets, or armies of robot computers, to inflict massive damage that could not be accomplished with individual computers alone. Who are these criminals and what motivates them? How can we stop them—and what might happen if we don''t?