Assessment for sex offenders with intellectual and developmental disabilities (IDD) has progressed significantly over the last 15 years. Developments have included assessment of cognitions that support offending, assessment of victim awareness, emotion including anger, anxiety, and depression, behaviour, quality of life, and relationships. There have been major developments in risk assessment with research on both static measures of risk and structured clinical judgements. Perhaps the most important progress has been made in the understanding of dynamic or immediate risk with factors emerging that have strong relationships with offending. These factors are more amenable to treatment and management. There has also been significant progress in the understanding of pathways to offending, developmental factors in offending, and self‐regulation. Developmental factors and quality of life emerge as significant factors in the establishment of offending careers. All of these developments are reviewed and evaluated in this chapter.