The development of comprehensive treatment services for intellectually disabled sex offenders has been slow in comparison with mainstream sex offender treatment services, which have now achieved Home Office accreditation within the National Probation Service. The author discusses some of the reasons for this failure to keep pace, focusing on the relative absence of an expertise spanning the fields of sex offender work and intellectual disabilities. Attention is focused on the particular challenges of targeting treatment, measuring change and evaluating programmes delivered to these individuals. The author reports on steps that have been taken toward overcoming these obstacles and makes recommendations about further work to be done. (Contains 1 table.)