Chapter 4 addresses ‘how is the problem treated?’. In other words, the intervention that will be applied to the target behaviour. The four sections of the chapter address: (i) defining the intervention and its theoretical underpinnings; (ii) describing the intervention in terms of the type of intervention, materials, and resources needed to implement the intervention, and procedural details; (iii) practitioner competence; and (iv) treatment fidelity and its measurement.