This chapter proposes two contrasting processes by which scientists come to a consensus, which are labelled as “spontaneous” and “deliberative”. The spontaneous process involves a consensus that develops rapidly and spontaneously among experts in an area. It is more likely to be seen with scientific questions that involve simpler causality and strong associations between variables. The development of a consensus is hidden from view and may lead an outside observer to think that the scientific facts emerge directly from the evidence. The deliberative process, on the other hand, is much slower, and it may take decades to come to a consensus. It is more typical with complex scientific questions where the evidence is extensive and involves multiple disciplines and methodologies. It involves formal methods to develop the consensus, such as consensus conferences, expert working groups set up by international scientific organizations, Delphi consensus studies of expert opinion and formal votes by groups of acknowledged experts. Deliberative consensus is becoming more important as scientists deal with increasingly complex problems in areas of global importance where coordinated action is required.