Attention Deficit Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder that starts in early childhood and continues throughout the lifetime. The main symptoms of ADHD are inattention and hyperactivity/impulsivity. Even though substance misuse is one of the main comorbid conditions of ADHD, it has been an exclusion criterion in most studies of ADHD. The limited number of studies on ADHD and substance misuse measured ADHD as a whole, but in this thesis, the two main symptoms of ADHD have been divided to investigate the relationship between each symptom and developing alcohol, cannabis, and nicotine use. Studies show that individuals with ADHD and comorbid substance misuse become drug addicted at a younger age, use higher amounts of drugs, relapse is more common among them and ADHD medication treatments are less effective in them. There are common personality traits and impairments in individuals with the symptoms of ADHD and those who use alcohol, cannabis and nicotine. The studies of this thesis investigated the role of different facets of Executive Functions (EF), facets of impulsivity, emotional regulation, bipolar disorder and sleep quality in the relationship between each ADHD symptom and alcohol, cannabis, and nicotine use in typically developing participants. The results of Study 1 showed that hyperactivity/impulsivity explained additional variance after accounting for the facets of EF. Study 2 showed that inattention explained additional variance over and above the facets of impulsivity. Additionally, in studies 3, 4 and 5, emotional regulation, bipolar disorder and sleep quality were partial mediators between each ADHD symptom and some alcohol, cannabis and nicotine use scores, which are presented in detail in the next chapters
