The precautionary principle - the environmental version of the admonition "first, do no harm" - is now enshrined in numerous international environmental agreements, including treaties addressing global warming, biological diversity and various pollutants. Some environmentalists have invoked this principle to justify policies to control or ban any technology that cannot be proven to cause no harm. In this innovative book, the author shows that the current use of the precautionary principle to justify such policies is flawed and could be counterproductive because it ignores the possible calamities those very policies might simultaneously create or prolong. The author develops a framework to evaluate policies for three contentious environmental policy issues facing humanity and shows that some of environmentalism's favourite policy prescriptions are in fact likely to do more harm than good.