Neurodevelopmental disorders (NDDs) including autism spectrum disorder (ASD), attention-deficit hyperactivity disorder (ADHD), and intellectual disability (ID) are clinically heterogeneous, often co-occur, affect ~15% of the EU population; and are associated with somatic illnesses (e.g. epilepsy, autoimmune and gastrointestinal disease) that lead to a significant increase in morbidity and mortality. For instance in ASD the combination of ID and epilepsy is associated with a reduction in lifespan of ~ 20 years; and an economic cost that is greater than cancer, stroke, or dementia. Yet, the research spend on NDDs is less than 1% of those disorders. Hence, we lack effective new treatments for NDDs and do not understand why they co-occur. There is hope, however. Recent evidence shows that rare genetic variants increasing risk for NDDs are shared, converge on final common pathways (e.g. synaptic plasticity, glutamate and GABA neurotransmission, and excitation/inhibition imbalance), and a key role is likely played by immune dysregulation. CANDY’ innovation is to test, for the first time, if NDDs, and their common mental and somatic multimorbidity, are caused by a combination(s) of common and rare genetic variants and immune activation acting at different ‘ensitive periods’ Our multidisciplinary team of world leading academics, patient organizations and SMEs will cost-effectively leverage existing EU-funded studies to 1) identify novel mechanisms underpinning NDDs and their multimorbidity, 2) develop new strategies for prevention and treatment, 3) deliver novel biomarkers to guide early diagnosis, stratification and/or treatment monitoring, and 4) provide open-access data bases, translational test batteries, and tools and targets for valorization. Together we will transform the landscape for people with NDDs and make possible personalized medicine approaches that target particular mechanisms, in specific subgroups of individuals and at different life stages.