Attention Deficit Hyperactivity Disorder (ADHD) has been considered as one of the most frequently diagnosed psychiatric disorders in children and adolescents. Nowadays it is becoming among one of the most common lifelong psychiatric disorders. Although ADHD symptoms are clearly defined by DSM 5, it influences the life of each person in many different and variable ways. ADHD is not a single phenomenon with simple causes and simple manifestations. One of the main characteristics of ADHD is variability. Variability in etiology, genetics, biology of the disorder, symptomatology and especially real life manifestation. To understand ADHD means to understand its high variability that is one of the most typical signs of this diagnosis. The present book approaches ADHD from the perspective of its variability. It reviews the variability of ADHD from etiology through somatic and neurobiological causes and manifestations to psychological and social context. The book provides a review of variability of the diagnoses also in very specific areas such as in the somatic development of ADHD children. The aim of this book is to provide an overview of the variability of ADHD for professionals in the broadest area of mental health as well as for related professions. The book is a great source for clinical as well as research workers.