This dissertation, which lies at the intersection of law and biosciences, explores ethical and legal implications of emerging biotechnology. More specifically, it focuses on the use of direct biological intervention for cognitive enhancement. Recent developments in neuroscience have enabled technological advances to modulate cognitive functions of the brain. Most of these brain intervention technologies were initially developed as treatments for neuropsychiatric diseases, but researchers have begun to investigate their potential to enhance cognitive functions in the healthy population. Despite ethical concerns about cognitive enhancement, both individuals and society as a whole can greatly benefit from these technologies, depending on how we regulate their use. To date, regulatory analyses of brain intervention technologies have focused on a technology itself -- for instance, the US Food and Drug Administration (FDA)'s regulation of a brain stimulation device -- rather than the use of a technology, such as the use of a brain stimulation device at work or school. Given that some forms of cognitive enhancement have already started to penetrate the general public's everyday life, we should launch a discussion about potential regulatory issues regarding their use in various real-world situations. The goal of the first paper is to fill the gap by providing an analytical framework in which to examine these regulatory issues. It aims to illustrate the issues around respecting autonomy and preventing coercive use of cognitive enhancement. The proposed framework categorizes the real-world settings where a brain intervention technology can be used for cognitive enhancement based on two criteria -- who is subjected to cognitive enhancement and who imposes cognitive enhancement. Based on this framework, the article analyzes regulatory issues arising out of every combination of subject/imposing party by taking one example case. Focusing on the regulations in the United States, this analysis demonstrates the current lack of adequate safeguards against coercive use. The existing consent requirement or appeals processes are not sufficient to prevent the use of cognitive enhancement against the subject's will. This paper concludes by calling for more attention from government agencies and researchers to develop sound protections against the coercive use of cognitive enhancement. This paper was published in Neuroethics in 2019. The second paper tackles one possible use of enhancement -- use of direct biological brain intervention to improve cognitive function in minor children. Parental use and state regulation of cognitive enhancement will inevitably cause tensions between parent, child, and state. These tensions stem from two different but fundamentally related causes: minors' incompetency to make decisions about their own welfare and parental autonomy from the state to make decisions about the upbringing of their minor children. However, these tensions are not without precedents. The courts have frequently struggled to set the boundary of parental autonomy and to balance parent's rights, children's interests, and state's interests and have accumulated extensive precedents in various contexts. This paper reviews previous US court decisions in the contexts analogous to cognitive enhancement -- medical treatment, education, and mandatory vaccination -- and analyzes their implications for future discourse on the use of cognitive enhancement on minors. The third paper is an empirical study on the mechanism of a brain stimulation technology called transcranial direct current stimulation (tDCS). tDCS delivers weak direct current to the brain using two electrodes attached to the scalp, and it is believed to be able to alter neuronal activation in a target region under/near the electrodes. It is gaining momentum both in the research community and in the general public as a safe and affordable tool for treatment and enhancement alike. However, recent meta-analyses have reported that there are inconsistent and conflicting results among tDCS studies. These inconsistencies stem in part from the lack of precise understanding of the mechanism underlying tDCS. Using a novel MRI technique, this study measures the primary direction and magnitude of current in a human brain undergoing direct current stimulation. The results show that current flow deviates significantly from its desired path under modeling and that the current primarily travels through cerebrospinal fluid, which has higher conductivity compared with gray matter and white matter. It also demonstrated that only some portion of injected current actually reached the cortex. This study provides a new way for in vivo investigation of tDCS current. Its findings also raise concerns about the ability of tDCS to target a specific region in the brain, which in turn suggests the risks of its premature application in and outside of a clinical context to modulate cognitive function. Given current and potentially more widespread use of cognitive enhancement, this dissertation should provide a useful guide for policymakers and researchers to identify issues around the use of direct biological brain interventions for cognitive enhancement and to develop sound policies to ensure the responsible use of these novel technologies.