ADHD medication of children and adolescents is becoming increasingly common. Clinical experience and scientific studies have proven that approximately 30% of children/adolescents with ADHD do not benefit from this treatment. However, there is insufficient knowledge about who these children are. All children and adolescents, who start treatment with ADHD medication at public Child and Adolescent Psychiatry units in Stockholm and on Gotland, will be asked to participate in the study. The investigators intend to monitor the patients´clinical symptoms and possible side-effects after treatment start. The investigators will collect background information and saliva samples from the patient and his/her parents to be able to study if there are any genetic (hereditary) or other markers that can predict positive or negative outcomes of the ADHD medication. With this information, the investigators aim at, to a greater extent, be able to individualize treatment choices for children and adolescents with ADHD without unnecessary, costly and possibly unfavorable treatment attempts.