Self-regulation is a multi-faceted construct that concerns goal-directed behaviors, which aid individuals in everyday life and in achieving long-term goals. Self-regulation in believed to progress in a hierarchical fashion, in that simple cognitive functions are integrated into more complex functions across development. Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous childhood-onset disorder, characterized by deficits in various aspects of self-regulation, including core symptoms of inattention and hyperactivity/impulsivity, and comorbidity with externalizing disorders such as oppositional defiant disorder (ODD). The last decade has seen a shift from simple etiological models towards more complex ones, which stress multiple pathways to the disorder. In addition, there is an ongoing search for early markers of the condition, which will increase our understanding, enable early detection, intervention, and perhaps even prevention of the full disorder. Cognitive regulation, aspects of temperament (i.e. negative affect, surgency, and effortful control), and parenting are three areas of importance for self-regulation in general and of ADHD symptoms in particular. Grounded in these three constructs, informed by the hierarchical model of self-regulation development, and a multiple pathway perspective on ADHD, the present thesis aimed to map development of self-regulation, with a special focus on inattention and hyperactivity/impulsivity. Four studies were conducted based on three samples, ranging from infancy to 12 years, including both typically developing children and children diagnosed with ADHD. Study I found that early sustained attention predicted later cognitive regulation, providing support for the hierarchical model of self-regulation development. In addition, maternal sensitivity contributed to higher levels of emotion regulation whereas surgency contributed to lower levels of emotion regulation. Study II gained support for a multiple pathway perspective on ADHD, in that higher temperamental regulation and maternal sensitivity contributed to lower levels of inattention and hyperactivity/impulsivity, and higher surgency contributed to higher levels of hyperactivity/impulsivity. Study III replicated findings from Study II and confirmed early temperament markers of later inattention and hyperactivity/impulsivity, and that early cognitive regulation was a poor predictor of later symptoms. Study IV proposed contributions of multiple regulatory functions to ADHD symptoms and elevated negative affect in ODD. The latter was moderated by parental support, which seemed to be a protective factor for children with high levels of negative affect. In all, the findings point to the importance of both intrinsic and extrinsic factors in the development of self-regulation, which seems to progress in a hierarchical fashion. Aspects of temperament rather than cognitive regulation seem to be valid early markers of later inattention and hyperactivity/impulsivity. Multiple pathways to ADHD symptoms are proposed, with contributions of maternal sensitivity and temperament early in development and different regulatory functions in school-aged children. In addition, elevated negative affect in combination with low parental support seem to be characteristic of ODD rather than of core symptoms of ADHD. The thesis contributes to the complexity and heterogeneity of ADHD and that ADHD is best viewed as a developmental disorder, in that the influence of various regulatory factors change over time.