Attention deficit hyperactivity disorder, ADHD, is one of the most common neurodevelopmental disorders that affects up to 5% of school-aged children. Despite the defined diagnostic criteria, we are not always able to make adiagnosis as quickly as possible and to implement optimal treatment. Despite different and advanced methods and technologies used to study ADHD, we still not fully understand the biological basis of attention deficit hyperactivity disorder. Therefore, research is continuing to explain genetic and neurobiological background of the disorder. Genetic analysis focuses on the search for risk genes (e.g., mutations, CNV polymorphisms), their transcripts and proteins as well all modifying molecules (epigenetic modifications). Not without significance is the search for non-invasive, simple and cheap peripheral biomarker assays, extremely valuable in the diagnosis, prediction, and monitoring of the disorder. In this review, we summarize current knowledge on abroad range of biological processes underlying ADHD. The results of the presented molecular and neuroimaging studies indicate research challenges and the possibility of clinical application of important genetic and non-genetic biomarkers related to ADHD.