Despite concerns about the growing threat posed by domestic radical environmental and animal rights groups to the United States, there has been little systematic quantitative evidence depicting the characteristics of their attacks over time. In this paper we analyze data on 1,069 criminal incidents perpetrated by environmental and animal rights extremists from 1970 to 2007. Based on the Global Terrorism Database’s definition of terrorism, we classified 17 percent of these incidents as terrorist. To supplement the analysis, we also conducted interviews with a non-random sample of twenty-five activists who self-identified as part of the environmental or animal rights movements. We find that overall, the attacks staged by radical environmental and animal rights groups thus far have been overwhelmingly aimed at causing property damage rather than injuring or killing humans. Further, results from our interviews suggest that activists appear to weigh carefully the costs and benefits of illegal protest. Despite the fact that attacks by environmental and animal rights groups have thus far been almost universally nonviolent, concerns linger that this situation might change in the future.