This thesis is a critical analysis of the law of self-defence in Scotland and England. It is argued that it is far more difficult to justify the use of lethal force in self-defence than is commonly assumed and that, drawing on the work of Uniacke, the most convincing justification is on the basis of a right to life, accompanied by a theory of forfeiture. That is, the victim of an attack is entitled to protect herself from an aggressor who threatens her right to life. The reason why she is permitted to kill the aggressor, but the aggressor is not permitted to kill her, is because an aggressor forfeits her right to life by virtue of becoming an unjust immediate threat to the life of another. However, the right to life is seen as a fundamental right that should be respected in relation to all human beings, even aggressors, as far as is reasonably possible. This is because, unlike almost all other types of loss, a deprivation of life is something from which the victim can never recover and for which the victim can never be compensated. As such, it is argued that an aggressor's right to life is only forfeited at the point where it is no longer reasonably possible to save both the life of the victim and the life of the aggressor. On this basis, the thesis proceeds to argue that the law of self-defence should contain strict rules on when it is permissible to take the life of another human being. As such, relatively restrictive rules are proposed in relation to five specific aspects of self-defence law: retreat, mistake, self-generated self-defence, imminence and proportionality. It is suggested that Scots law conforms to these strict rules to a greater extent than does English law.