The online marketplace for guns is vast -- and growing. Each year, millions of people connect through online ads to buy and sell firearms. And because many of the transactions are conducted by so-called 'private sellers' who are not required by federal law to conduct background checks, guns routinely change hands with no questions asked. In the digital age, convicted felons, domestic abusers and other dangerous people who are legally barred from buying guns can do so online with little more than a phone number or email address. And they do. Countless tragedies have demonstrated that determined criminals are exploiting this 'private sale loophole' to acquire guns online and murder innocent people The National Rifle Association, which once supported the National Instant Criminal Background Check System (NICS), has recently opposed efforts to close this growing loophole. One argument often recurs: criminals won't submit to background checks. This report demonstrates that their claim is both false and true. Criminals undeniably do submit to background checks: in 2010 alone, federal and state checks blocked more than 150,000 gun sales to prohibited buyers. But criminals also undeniably avoid background checks -- by exploiting the private sale loophole. Indeed, one measure of NICS's success is that it appears to have forced a growing number of criminals to seek out private sellers since the system was established in 1998.