We identify the patents litigated most frequently between 2000 and 2007, and compare those patents to a control set of patents that have been litigated only once in that period. The results are startling. The most litigated patents are far more likely to be software and telecommunications patents, not mechanical or other types of patents. They are significantly different from once-litigated patents in ways that signal their value up front. And they are disproportionately owned by non-practicing entities (aka trolls). The results don’t answer all the policy questions; we offer only one important piece of a larger mosaic. But they have significant implications for debates over patent reform, since we show both that the most litigated patents are the most valuable ones and that they are most commonly in the hands of companies other than the ones building new products.