Although attention deficit - hyperactivity disorder (ADHD) has a distinct developmental progression and a substantial genetic component, very little is known about the genetic contribution to its development and the question of whether the same genes contribute to ADHD throughout childhood and into adolescence. Data from one of the most extensive twin databases, the Australian Twin ADHD Project (ATAP) have shown that ADHD is inherited as a behavioural dimension rather than as a discrete disorder. Behavioural genetic analysis is used in this study with both cross-sectional and longitudinal analyses of inattention and hyperactivity - impulsivity symptoms of ADHD to demonstrate that much of the consistency in behaviour during childhood and early adolescence is due to genetic influences. Genetic analyses help identify environmental influences that impact upon symptoms of inattention and hyperactivity- impulsivity. It is emphasised that the main function of genetic analysis does not lie in simply estimating the heritability of a trait, but rather in contributing to the understanding of developmental progression and environmental factors that may impinge upon this.