Scientific knowledge—a species of collective knowledge—contributes greatly to human well-being; yet scientific knowledge enables technologies that can be extremely harmful. Accordingly, the question arises as to whether we ought to aim at ignorance and, in particular collective ignorance, rather than scientific knowledge, of certain technologies. We might do this by means of banning certain scientific research, e.g. into biological weapons, and/or by censorship of certain scientific findings. In this chapter I provide a taxonomy of concepts of collective knowledge (e.g. public propositional knowledge, expert practical knowledge) and an account of the related concepts of collective ignorance. In doing so my concern is with the concepts of collective knowledge and ignorance relevant to harmful technology and, especially, scientific knowledge/ignorance in the chemical industry, nuclear sciences, cyber-technology field and biological sciences relevant WMDs—such knowledge/ignorance being salient in discussions of dual use issues.