Judges often speak of “freedom of thought” as a liberty central to American constitutional jurisprudence. But why does thought need protection even when it remains unexpressed in speech and hidden? This chapter explores two possible answers. One is that understanding a principle of freedom of thought explains why speech is strongly shielded by the First Amendment of the US Constitution: Our thought, judges and scholars stress, is central to how we define ourselves and speech is the key means of conveying thought and shaping it. Yet a right to freedom of thought might also stand on its own. The Court’s 1969 decision in Stanley v. Georgia provides two possible accounts of how it might do so. The first reaffirms and modestly expands the long-standing principle that officials may not target and punish us solely because of our thoughts. The second reading of Stanley goes further: It protects us not only against state action aimed at controlling thought but also that which interferes with certain environments or resources that allow us to shape our thought. This second account, the chapter explains, is more suited than the first to address certain challenges raised by emerging neurotechnologies.