This paper investigates the role of caregiver mental health and parenting practices as predictors of attachment in children with intellectual disability/developmental delay, comparing between children with ASD (n = 29) and children with other developmental disabilities (n = 20). Parents reported that children with ASD had high levels of anxiety and stress, and attachment insecurity in children (less closeness and more conflict in attachment relationships, and more inhibited attachment behaviours) compared with children with other developmental disabilities. Children's attachment quality was associated with parenting practices and the presence of an ASD diagnosis. These results highlight the bidirectional nature of the quality of caregiving environments and attachment in children with ASD, and also provide a strong rationale for targeting children's attachment quality in early interventions. [This article represents a collaborative effort on behalf of the Mental Health of Young People with Developmental Disabilities (MHYPeDD) team.]