When it opens in December, the Wallis Annenberg Wildlife Crossing will be the largest in the world. Stretched over a 10-lane freeway, it is intended to help save California's mountain lion population, which has become fragmented by the highway. It could also serve as a model for other projects, as crossings of this kind have often been viewed as too large to be effective. But will the crossing work? We examine the construction and the existing scientific data to find out.