This book addresses the morality of engaging in weapons research, a topic that has been neglected but which is extremely important. It is argued that this activity is both morally wrong and morally unjustifiable, and this implies that moral persons should not engage in it. The argument is not based on any pacifist assumptions: it is not assumed that neither individuals nor states should not defend themselves. What is wrong with weapons research is that it is the first step in the production of weapons, weapons are the means to harm, and harming without justification is always wrong. Those who study science, for instance those who are interested in the responsibilities of the scientist, are given a new perspective, while those who are practicing scientists will realize that they should not consider working to design new or improved weapons systems. This book is of interest to students and researchers working in ethics and technology, philosophy of technology, military ethics, and history of technology.