What explains the likelihood that a nongovernmental organization () will turn to the courts to pursue their policy goals? This article explores the factors that influence the mobilization of law by environmental s in four Western European countries. It finds that explanations focused on legal opportunity structures are unable to account for the patterns of within-country variation in legal mobilization behavior. The research also shows that bird protection s as well as home-grown national environmental s are generally more likely to turn to law than transnational environmental groups. Although resources and legal opportunities clearly matter to some extent, the author suggests—drawing on sociological institutionalist theory—that explanations of legal mobilization should (a) incorporate an understanding of how groups frame and interpret the idea of “the law” and (b) explore the role of “strategy entrepreneurs” who promote the use of particular tactics within an organization.