Although effective, humane treatments exist for persons with intellectual disabilities (ID) who have challenging behaviour, little research has examined the extent to which clients receive formal, documented vs. undocumented interventions. Caregivers (of 625 persons with ID living in community and institutional residences in Ontario, Canada) were interviewed to examine the prevalence of different types of interventions. Overall, 55% of the 2506 different interventions (for 1464 target behaviours) were informal (i.e. lacking documented input from a professional, written intervention plans, and systematic evaluation). No significant differences emerged on formality of intervention across participant gender, age, level of ID, and type of residence. There were significantly more informal than formal behavioural interventions and counselling psychotherapy, and no significant difference in the overall prevalence of formal and informal intrusive procedures. Behaviour control medications were paired more often with formal (67%) than informal interventions for dangerous behaviours. Formal interventions were associated with higher caregiver-reported estimates of behavioural improvement, higher inter-rater agreement on the descriptions of an individual client's target behaviours and interventions, and more caregiver training and supervision. The low levels of intervention accountability, training and supervision may place many clients with challenging behaviour at increased risk for ineffective and unnecessary restrictive interventions, and physical abuse. The results of this survey stimulated the government to write province-wide standards (that have yet to be implemented).