OVER the years, the fight against online piracy has led to countless lawsuits by media companies and to escalating levels of law enforcement, all with mixed results.

Lately, though, new attention has turned to an aspect of online commerce that critics say finances online piracy: advertising.

Prodding from the White House and a recent academic report have put pressure on the online advertising industry to prevent ads — for jeans, say, or car insurance — from appearing on a page offering a free download of Season 2 of “Game of Thrones.” Yet these efforts have also been slow to produce results, in part because of the complexity of the online ad system.

This month, the University of Southern California’s Annenberg Innovation Lab released a report that ranked 10 ad networks on the amount of business they do with sites suspected of engaging in piracy, with Google and Yahoo placing high on the list. Ad networks use advanced computer algorithms to place ads on Web sites. They can be run by agencies, publishers or others.