When and why will organized interests choose to lobby the U.S. Supreme Court by submitting amicus curiae briefs? This article argues that organized interests are most likely to lobby the Court when conditions increase their expectation of influencing the Court’s policy outputs. However, when deciding which policy venue to lobby, organized interests that rely on membership support will also have to consider the effect of their lobbying decisions on their ability to attract and retain members. Analysis of the amicus curiae brief filings of 579 organized interests suggests that an interest is more likely to submit amicus briefs at the Supreme Court when the Court is receptive to the positions advocated by the interest and the interest has participated at the Court in the past. The results also indicate that membership-based groups will take into account the extent to which relevant cases have been covered by the media when choosing whether to lobby the Court.