Background: Interventions for challenging behaviours in individuals with intellectual disabilities benefit from outcome monitoring that takes clients' preferences into account. We determined clients' and representatives' preferred outcome domains and measures to secure their involvement in treatment decisions for challenging behaviours. Method: We used an inclusive Delphi method. A focus group of individuals with mild intellectual disabilities and representatives of those with moderate and severe intellectual disabilities prepared the first round by assisting us in collecting possible outcomes. Panels of individuals with intellectual disabilities and representatives were composed to achieve consensus on instruments for preferred outcome domains. Results: Preferred outcome domains were behaviour, side-effects of psychotropic drugs, quality of life, daily functioning, caregiver burden and family quality of life. Corresponding outcome measures included self-report, interview and proxy-scales, including spoken versions. Conclusion: Including the preferred domains on outcomes of interventions for challenging behaviours is recommended. Research on corresponding outcome measures is necessary.