Hyperactivity syndromes and disorders (ADHD and HKD) include the symptoms of overactivity, inattention, and impulsivity, which occur in many other mental disorders as well, including mental retardation (MR). It is not surprising that symptoms of ADHD occur significantly higher in children with learning disabilities. Dekker and Koot (2003) found a prevalence of 14.8 % for ADHD in Dutch children attending special schools, and Emerson (2003) reported rates of 8.7 % for HKD in children with global learning disability, representing a 10-fold increased risk compared to the prevalence of hyperactivity (0.9 %) in the general population sample. Yet only very few studies have been published concerning ADHD in children with mental retardation. Several features distinguish the diagnoses of ADHD and MR. In contrast to the limited knowledge about the differences and similarities of ADHD and MR, many studies considered stimulant medication as a pharmacological management strategy for children suffering from ADHD, MR, or both. According to these studies, psychostimulants may improve the target symptoms of hyperactivity, impulsivity, disinhibition, and inattention, albeit with caveats: ADHD symptoms in patients with MR may be less responsive to medical treatment than in patients without MR. Moreover, people with MR may be more susceptible to side effects.