The right to freedom of thought represents as fundamental a human interest as do rights like privacy and dignity. However, unlike these rights, freedom of thought has received comparatively little attention. This paper seeks to redress this by arguing for an expansion of what freedom of thought entails. Specifically, it argues that freedom of thought should be understood as entailing 'cognitive liberty', which has both a positive and a negative dimension. The positive dimension entitles one to self-determine the contents and processes of one's thoughts (that is, to control one's own brain chemistry); while the negative dimension protects one from direct and non-consensual interventions into one's mental states (whether done by a state or a third party). This expansion of freedom of thought is increasingly necessary as both our understanding of the human mind and our capacity to interface with it expand. This is established by reference to two case studies: the injustice of international drug policy, and the potential of neurotechnology. In each case, the focus is on how cognitive liberty, if taken seriously, creates radical new possibilities for regulation. Specifically, it can facilitate drug policy that is just and bring coherence to the regulation of neurotechnology.