IMAGINE-ID is a national longitudinal cohort study that investigated the genetic causes of intellectual disability and how genetic changes affect children and young people's behaviour. Between 2014 and 2019, the study recruited participants aged between 4 and 19 years who met all four of the following criteria: i) having an intellectual disability; ii) having had a diagnostic microarray by an accredited Regional Genetic Centre, or next-generation sequencing; iii) having at least one Copy Number Variant or Single Nucleotide Variant reported as pathogenic; iv) having a legal guardian available to consent and provide detailed medical and behavioural history. Participants were divided into two cohorts; the UCL sample included over 3,400 participants, and the Cardiff University sample included 520 participants. The Cardiff sample was followed up twice, at baseline between 2014 and 2018 and once more between 2020 and 2024. The UCL sample was followed up six times, at baseline between 2014 and 2019, and in 2020, 2021, 2022, 2023, and finally in 2024.