How can researchers appropriately estimate the number of extremists, particularly violent extremists, in Canada? Having a reasonable idea of the size of the threat posed by violent extremists is a central policy consideration. Crucially, accurate estimates would allow for the better allocation and prioritization of limited resources. As well, they would allow afford a better understand the evolving nature of the threat posed by terrorism. Finally, to that extent that various policy initiatives are aimed at preventing or reducing radicalization, knowing the number of terrorists provides a baseline against which these programs may be evaluated.