Our understanding of the risk assessment of terrorist and extremist prisoners is in its infancy, yet this is clearly a critical issue. How can one tell if a prisoner is still dangerous or not? What are valid measures to assess risk and what type of evidence is worth examining? In considering risk assessment of terrorists and extremists in prison settings there are a range of essential issues to consider. Crucially, not all terrorist and extremist prisoners are the same and factors which apply clearly to one individual will not apply equally strongly to others. As a result, effective risk assessment processes need to be nuanced. This chapter highlights the range of issues which are likely to be of most use when considering risk assessment in the context of a terrorist prisoner, and also identifies what sources of information can provide the necessary insight to inform this assessment