Both within and outside of the academy, most commentary treats neural engineering, and the “crisis of agency” it allegedly introduces, as something new. Yet my dissertation shows that technologies to modulate and record brain activity have been (1) a key factor in the creation of the discipline of neuroscience, (2) a central concern in the development of liberal ideologies of personhood and freedom, and (3) a critical feature of the what scholars have recently termed the “data economy” or “surveillance capitalism.” My work makes these arguments by offering the first monograph length scholarly study of neural engineering, documenting the rise, fall, and reappearance of brain implant technology. Techniques to stimulate and record the human brain were at the core of the creation of the discipline of neuroscience, and after 1951, long-term brain implant systems were used for research in dozens of human patients. The public backlash against brain engineering was enormous, allying both conservatives and New Left in the latter half of the 1960s, and by the late 1970s the once thriving research field had disappeared. But brain implants were not gone for good: in 2013, President Obama announced the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative to map the brain with a wireless brain implant called Deep Brain Stimulation (DBS) hailed as its flagship technology. While brain implants in the 1970s seemed to threaten the existence of an autonomous self on which both the free market and democracy were premised, brain implants in the era of big data promise dazzling economic value. The global brain stimulation market is projected to be worth $6.2 billion by 2022, much of it underwritten by US military funding, with data monopolists like Alphabet and Facebook vying with Elon Musk to establish brain implant labs. In contrast to prevalent commentary that takes brain implants to have introduced a crisis of the agentic human, I argue that neural engineering’s reappearance has been made possible by permutations in liberal ideologies of “freedom” and “the human”; changes which I argue are conditioned by rise of post-industrial or “data” capitalism. As a critical account of the neurosciences, the story my dissertation tells differs from the many accounts of “neurosubjectivity” currently influential in STS, which date changes in liberal conceptions of the human to the ascent of neuroscience in the 1990s. Instead, the work shows how the politics of “freedom” in the latter half of the twentieth century came to be defined against neural engineering as a foil. It documents how debates about brain implants played a central role in constructing key liberal political concerns; among them bioethics, privacy law, and the legal construction of the body as private property. These changes directly undergirded the subsequent development of the neurosciences, chiefly the Reagan-era fusion of the US academy with the biotech industry. In this way, the project shows that the 1970s defeat of neural engineering ramified into the necessary conditions for its re-emergence at the cutting edge of “data capitalism.” Ethnographic research for the project began in 2014, and I spent a total of 34 months in four labs in the US and France developing DBS for psychiatric disorder. 14 of those months were in a lab whose project was funded by DARPA, the experimental sciences branch of the US military. Archival research was conducted at 20 institutions. The dissertation begins in the 1930s by charting the network of neurophysiologists who would constitute the brain as an electrical organ. These neurophysiologists would form the first organization to call its object “neuroscience,” the International Brain Research Organization (IBRO), funded by UNESCO in 1958 to bring together Soviet and Western neurophysiologists. IBRO’s aim was to construct a universal science of the brain, one that could resist the deforming hand of the US military state’s control of science. But IBRO would be defeated by the militarization of neuroscience in the late 1960s, and its progressive imagination for neural engineering would become a bête noir for the 1970s politics of freedom. The second section offers an intellectual history of the response to neural engineering 1951-76, from midcentury panics about “totalitarian” mind control to the New Left’s mobilization against “psychiatric technocracy.” I recount the key role of brain implant research in the 1972-76 Congressional hearings that would produce the fields of bioethics and medical privacy law, while causing neural engineering to disappear. The “reinvention” of DBS hinged on its application to movement disorders like Parkinson’s in the 1980s, which allowed scientists to frame the technology as one that restored, rather than violated, individual agency. The final section begins with 2005 research “breakthrough” that began investigating—“for the first time”— brain stimulation for psychiatric disorder. Drawing on my ethnographic work, the last section follows DBS research from clinical trials to biotech startups, mapping emerging conceptions and practices of selfhood, agency, and value production. I show how these practices of scientific experimentation rely on liberal contractual forms: an agentic “self” who “consents” to experimental procedures, even while the data produced by these experiments are reframing key concepts like “decisions,” “agency,” and “ownership.” I argue that this turn toward the data-productive brain does not signal the end of a politics of free will; rather, the new forms of “surveillance” capitalism enabled by brain implant research rely on strategically invoking the agentic liberal subject through legal forms like the contract and bioethical “consent.”