Apple is the patent trolls’ No. 1 target, with 171 suits since 2009 by Philip Elmer-DeWitt @FortuneMagazine August 28, 2013, 1:05 PM EST E-mail Tweet Facebook Google Plus Linkedin Share icons Click to enlarge. FORTUNE — Buying patents and suing companies for violating them is a large and growing business, and nobody has felt its impact more than Apple AAPL . Of all the companies sued over the past five years by so-called non practicing entities (NPEs) — a.k.a. “patent trolls” — Apple got hit the most: 171 patent lawsuits as of June, according to PatentFreedom, a community of companies that tracks the activity of 710 parent NPEs and their 2,500 subsidiaries and affiliates. Among the fattest targets, Apple was followed by Hewlett-Packard (137 lawsuits), Samsung (133) and AT&T (127). (See list below.) The targets of these lawsuits tend to view patent trolls as technological parasites. NPEs, by definition, don’t actually manufacture anything. Instead, they either develop a process and patent it, or they buy or license the patents from others. “Ultimately, a patent is nothing more or less than a license to sue someone,” Michael Brody, an intellectual property specialist at Winston & Strawn, told an audience at Stanford University earlier this week. An account of Brody’s talk posted Wednesday by TechHive‘s Mark Hachman is packed with trollish facts and figures: About 125,000 software patents are filed annually. “And that kind of meat has attracted sharks,” writes Hachman. In 2012, more than 4,200 separate companies or individuals were sued by NPEs. The average licensing cost for a case that goes to trial is $7.5 million The average licensing cost for an out-of-court settlement is $29.75 million. The cost to defend each suit ranges from $800,000 per suit for startups to an average cost of $7.9 million for the those firms with over $50 billion of annual revenue. In total, the annual cost or “troll tax” for defending NPE suits is $1.04 million annually for firms with annual revenue under $1 billion, and up to $57.67 million for those with revenues over $50 billion All told, an NPE has a 24.1% chance of “winning,” either by negotiating a settlement, winning at court or winning at appeal. The “net discounted value” of an NPE suit is $800,000; i.e., it’s likely that the suit itself will net at least that much just by being filed. “That’s a good business to be in,” says Brody. “And that’s why a lot of people are in it.” Below, PatentFreedom‘s list of the most pursued companies.