Why do interest groups participate in state courts of last resort by filing amicus curiae briefs? Are they influential when they do? This dissertation examines these questions using an original survey of organized interests that routinely participate in state supreme courts, as well as data on all amicus curiae briefs and majority opinions in over 14,000 cases decided in all fifty-two state supreme courts for a four year period. I argue that interest groups turn to state judiciaries to achieve the dual goals of influencing policy and organizational maintenance, as amicus briefs can help organized interests achieve both outcomes. Furthermore, I contend that amicus briefs are influential in shaping judicial policy-making through the provision of legally persuasive arguments. The results suggest that interest groups do file amicus briefs to both lobby for their preferred policies and to support their organization's long-term viability. Additionally, the results indicate that organized interests also participate in counteractive lobbying in state courts of last resort by filing amicus briefs to ensure their side is represented and to dull the effect of oppositional amici. The findings also demonstrate support for the influence of amicus briefs on judicial policy-making on state high courts, as amicus briefs can influence the ideological direction of the court's majority opinions. Overall, this research extends our understanding of interest group lobbing in the judiciary and in state policy venues, and provides insight into judicial politics and policy-making on state courts of last resort.