Attention-Deficit Hyperactivity Disorder (ADHD) has symptoms of inattention, hyperactivity, or both. However, patients with ADHD often experience symptoms of other disorders such as autism spectrum disorder (ASD), epilepsy, conduct disorder, or anxiety symptoms. Previously, it was reported that there are shared heritability and cognitive process across the conditions, implying that there are common underlying biopsychological factors that have not been identified.Epigenetic biomarkers, especially blood DNA methylation (DNAm) became significantly important in understanding neurodevelopmental disorders such as ADHD. Even though the associations between DNAm and each condition were investigated, there is a limited amount of research on DNAm biomarkers across ADHD-related phenotypes (ADHD and co-occurring symptoms). We hypothesized that the underlying biological factors would be specific to clusters of conditions and could be used as a diagnosis factor for children. Therefore, we propose to investigate the relationships between each of the individual traits linked to ADHD and DNAm to reveal their epigenetic similarities and differences. The epigenetic similarities and differences will give in-depth insight to understand ADHD. We will use a series of computational methods including EWAS and machine learning to investigate the DNAm biomarkers of complex ADHD-related phenotypes from around 1500 participants from the Avon Longitudinal Study of Parents and Children.