It is proposed that the time is ripe for the development of secondary preventive interventions for attentiondeficit/hyperactivity disorder (ADHD). By targeting preschool children, a developmental stage during which ADHD symptoms first become evident in most children with the disorder, many of the adverse long-term consequences that typify the trajectory of ADHD may be avoided. A dynamic/ interactive model of the biological and environmental factors that contribute to the emergence and persistence of ADHD throughout the lifespan is proposed. Based on this model, it is argued that environmental influences and physical exercise can be used to enhance neural growth and development, which in turn should have an enduring and long-term impact on the trajectory of ADHD. Central to this notion are 2 hypotheses: 1) environmental influences can facilitate structural and functional brain development, and 2) changes in brain structure and function are directly related to ADHD severity over the course of development and the degree to which the disorder persists or remits with time. We present experimental and correlational data supporting the first hypothesis and longitudinal data in individuals with ADHD supporting the second. The case is made for initiating such an intervention during the preschool years, when the brain is likely to be more "plastic" and perhaps susceptible to lasting modifications, and before complicating factors, such as comorbid psychiatric disorders, academic failure, and poor social and family relationships emerge, making successful treatment more difficult. Finally, we review recent studies in young children with ADHD that might fall under the umbrella of secondary prevention.Keywords ADHD . prevention . environmental enrichment . development . early intervention . preschool.Attention-deficit/hyperactivity disorder (ADHD) is an early emerging neurodevelopmental disorder that persists into adolescence and early adulthood for a substantial portion of afflicted individuals [1]. In addition to the "core symptoms" of inattention, impulsiveness, and hyperactivity that define the disorder, a substantial portion of individuals diagnosed with ADHD present with an array of neurocognitive deficits, psychiatric comorbidities, and social and emotional difficulties [2,3]. As a result of persisting impairments across an array of behavioral domains, long-term outcome for many, if not most, children with ADHD is far from optimal [4].There are several evidence-based pharmacological and psychosocial interventions that are effective for treating the core symptoms and, to a lesser extent, the associated features in youth with ADHD. Both stimulant [5] and nonstimulant [6] medications reduce the severity of core ADHD symptoms, as well as associated oppositional defiant behaviors. Similarly, parent-and school-based behavioral interventions have