Repetitive behavior is a core symptom of Autism Spectrum Disorder (ASD) and Obsessive-Compulsive Disorder (OCD), and has been associated with impairments in cognitive control.However, it is unclear how cognitive control and associated neural circuitry relate to the development of repetitive behavior in children with these disorders. In a multicenter, longitudinal study (TACTICS; Translational Adolescent and Childhood Therapeutic Interventions in Compulsive Syndromes), the development of cognitive control was assessed during late childhood using a longitudinal fMRI design with a modified stop-signal task in children with ASD or OCD, and typically developing (TD) children (baseline: N=122 (8-12y), follow-up: N=72 (10-14y), average interval: 1.2y). Stop-signal reaction time (SSRT) decreased over development, regardless of diagnosis. Repetitive behavior in children with ASD and OCD was not associated with performance on the stop-signal task. There were no whole-brain between-group differences in brain activity, but ROI-analyses showed increases in activity in right precentral gyrus over development for children with OCD. In sum, even though subtle differences were observed in the development of brain activity in children with OCD, the findings overall suggest that the development of cognitive control, as assessed by the stop signal task, is similar in children with and without ASD or OCD. At baseline, we were able to include data for 122 participants between 8 and 12 years (ASD = 38, OCD = 23, TD = 61) (details are described in Gooskens et al., 2019). Ninetythree participants (ASD = 34, OCD = 15, TD = 44) participated in a follow-up visit.Participants' reasons for drop-out were mainly loss of interest, or wearing dental braces which prevented them for undergoing MRI assessment. ) and were commissioned by a multicenter study (COMPULS: Naaijen et al., 2016) as part of the overarching TACTICS collaborative initiative (http://www.tactics-project.eu). Exclusion criteria for all participants were an estimated total IQ below 70 and insufficient comprehension and speaking ability of the native language of the country. For MR scanning, the presence of metal objects in the body (i.e., pacemaker, dental braces), neurological illness or other contra-indications for MRI-assessment were exclusion criteria. Participants were asked to abstain from stimulant medication 24 hours before scanning. For both diagnostic groups, a concurrent diagnosis of the other disorder was an exclusion criterion (i.e. comorbid OCD for a child with ASD, or vice versa). In TD participants or their first-degree relatives the presence of any psychiatric diagnosis was an exclusion criteria. After description of the study, parents of all participants gave written informed consent.