The concept of participation is now widely accepted in healthcare and social services, but is less apparent in the delivery of services for vulnerable people who encounter the Criminal Justice System and its associated agencies (Buck et al, 2020). This article considers the extent to which children and ‘vulnerable’ adults who have been detained in police custody are currently able to actively participate in the design and delivery of Appropriate Adult Schemes. This paper draws together responses from 43 services in England and Wales concerning levels of service user engagement to ascertain the current level of participation, and to make recommendations for the future. The paper applies Forbat, et al.'s (2009) models of service user involvement to conceptualise ideological drivers which underpin the Appropriate Adult Scheme's commitment to involvement. The data reveal that while there is a genuine desire to improve service user participation, that institutional, financial, and attitudinal barriers mean that participation either does not occur or is at a very cursory level.