Background: Motivational interviewing is a promising method to increase treatment motivation for people with mild intellectual disability and challenging behaviour. The purpose of the present study was to identify how professionals could adapt motivational interviewing techniques for use with clients. Method: We conducted semistructured qualitative interviews and focus groups with 26 clients, parents, and professionals. A general inductive approach led to the identification of multiple core themes. Results: The authors recommend several modifications to accommodate motivational interviewing for use with clients: adapt to language level, adjust to cognitive abilities, and control for social desirability of responding. In addition, certain characteristics of professionals were also found to be critical for effective motivational interviewing: trustworthiness, engagement, acceptance, empathy, and honesty. Conclusions: Concrete recommendations for the adaptation of the motivational interviewing techniques for use with people with mild intellectual disability and challenging behaviour are identified. Certain characteristics of professionals are also critical for maximising the treatment motivation of clients.