This chapter provides a taxonomy of concepts of collective knowledge and an analysis of the related concepts of collective ignorance. In doing so, the concepts of knowledge and ignorance most relevant to harmful technology and, in particular, knowledge/ignorance of the technology of weapons of mass destruction (WMDs). Note that doxastic ignorance has no clear analogues in cases of acquaintance-knowledge or practical-knowledge. Dual-use technology (DUT) is technology that is developed in order to provide benefits to humanity but is, nevertheless, potentially very harmful. Scientific and technological knowledge is comprised in part of the propositional, acquaintance and practical knowledge of individual scientists and engineers. Let us distinguish between natural, institutional, and moral responsibility and, in respect of responsibility, between individual and collective responsibility. An agent, A, has natural responsibility for some action, x, if A intentionally did x for a reason and x was under A's control.