The progress of scientific technology means there is an increasing risk that terrorist groups, or even single individuals, will acquire a capacity to construct weapons of mass destruction. It might be possible for a well-organized terrorist group now or in the imminent future to manufacture a nuclear bomb. It is even easier to create biological weapons of mass destruction. The multi-culturalism of current liberal democracies makes it likely that they will contain groups of people hostile to their ideology. To counteract the threat of highly destructive attacks from such groups, liberal democracies have to avail themselves of the sophisticated means of surveillance that modern technology offers. Such surveillance will make these democracies less liberal, but it is possible to justify because there is no privacy right that it infringes. The xenophobia that results from a terrorist attack from some ethnic group is an even greater threat to liberalism.