The under-regulation of the private security industry has increasingly become a topic of media and academic interest. This Adelphi Paper enters the debate by explaining why the industry requires further regulation, and what is wrong with the current system. It begins by briefly defining the industry and explaining the need for more effective regulation, before analysing three types of regulation: domestic, international and informal (including self-regulation). The paper makes several unique contributions. First, it argues that no regulatory system dealing with private security companies can ever be perfect. The nature of the industry means that it will be inherently difficult to regulate. As a result, it is important to enhance all three types of regulation as much as possible, as opposed to creating model domestic or international schemes. Second, it takes the option of self-regulation seriously. While on its own self-regulation is unlikely to regulate the industry sufficiently, it has an important role to play alongside other forms of regulation. In particular, self-regulation enabled by legislation could be an effective means of regulation. Third, the paper argues that it is essential to treat mercenaries and private security companies as different problems requiring different regulatory solutions. Any form of international regulation attempting to deal with both mercenaries and private security companies will fail to deal with the real threat posed by mercenaries and fail to institute controls over an important and growing industry.