I analyze the case of humanitarian pro-migrant activists in southern Arizona between 2000 and 2010 to explore how contending groups wield law and legality claims in a dynamic policy environment. Humanitarian activists both evade and engage the law. They appeal to a higher law to elude charges that they are acting illegally, while seeking assurances that their actions are within the law. Law enforcement agents rely on the authority and technical neutrality of the law in redefining humanitarian aid as illegal, while expanding their own claims to carry out humanitarian work. This case study of advocacy on behalf of “illegal” migrants highlights how both activists and those who enforce the law redefine legality in strategic ways.