This paper explores the construct validity and structural properties of the ERG22+ formulation tool, used in the assessment of risk and need in extremist offenders. The ERG22+ was developed from a clinical perspective, and was based on the small number of case studies and limited research evidence that were available at the time. It suggests three domains relevant to risk, namely Engagement, Intent, and Capability. These were originally offered as a guide to risk formulation within a functional analysis framework, but were not empirically derived. The items within the ERG22+ are scored on ordinal ratings which offers the possibility to develop subscales within the tool, but the conceptual overlap across the domains offered suggests that these would not be empirically reproduced. In this paper the structural properties of the ERG22+ were examined through factor analysis and multidimensional scaling (MDS) to inform the development of measurement scales. The ERG22+ shows promise as a risk and need formulation tool, which could be developed further in light of the findings from this study. Issues of content validity and areas for further development are discussed.