This paper sets out the case for the manualization of sex offender treatment. The movement towards evidence-based practice in psychotherapy has led to strongly voiced opposing views on the value of manualization. However, "what works" evidence, i.e. the meta-analytical research behind the Risk-Needs-Responsivity model of offender rehabilitation, as well as the broader psychotherapy literature, demonstrates that manualized treatment is usually more effective. Manualized treatment is also more likely to be replicable and focused upon criminogenic targets. Arguments against manualization include the claim that it restricts therapeutic artistry, but this is not necessarily a bad thing. This paper also proposes some important characteristics of a good manual, which will ensure that treatment is structured and targeted appropriately, but which will not override the importance of a constructive therapeutic relationship between treatment provider and offender. The review concludes that carefully designed manuals are essential for sex offender treatment to be demonstrably successful, and for successful treatment to be replicated.