This article offers a first academic evaluation of the Special Demonstration Squad and the National Public Order Intelligence Unit, two British undercover police units working for the Metropolitan Police Service at different times between 1968 and 2011. It provides a historical overview of their infiltration of political groups involved in protest for the purpose of gathering criminal and political intelligence aimed at preventing violence, public disorder, and subversion. It discusses the controversies surrounding these units, and the related institutional responses, and offers an attempt at understanding their operations within the remit of intelligence-led policing and against a political culture that prioritizes action over inaction in reducing risks and threats to the State and society.Much has been written in the media about the infiltration of political groups involved in protest by British police units since 1968. Since March 2010, when The Observer 1 ran the first full story of a former undercover police officer ("Officer A," later identified as Peter Francis) working for the Special Demonstration Squad (SDS), The Guardian 2 has led an expos e of both the SDS and the National Public Order Intelligence Unit (NPOIU) and has unveiled the extent to which these two undercover units 3 gathered preemptive intelligence ranging from the clear prevention of political violence to the collection of information on more peaceful and less disorderly political groups. 4 A series of official investigations on, and reviews about, these two units have been carried out and are discussed in this article. The main aim of this article is to provide a first, exploratory academic study of the SDS and the NPOIU and to serve as a background study for future research on these two units and on the remits and limits of undercover policing within democratic societies. The first