It seems to be the perfect crime: a good four percent of all worldwide advertising expenditure on the Internet goes to fraudsters - however, the perpetrators rarely have to fear punishment.
Many companies have given up, but some start-ups are taking up the fight.
Clemens Zocher grabs his laptop and smiles: "Now I'll show you how to commit advertising fraud on the Internet - it's relatively easy," he says.
"No other crime is so lucrative and so risk-free."
Zocher is a technology expert at the German branch of the New York company Integral Ad Science. In fact, his daily job is to prevent internet advertising fraud. Now he's switching roles.