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52 Rohde, Buitelaar, Gerlach & Faraone Elementary school teachers might have the advantage of knowing well nor- mative behavior for the age-group, and to spend lots of time with children du- ring activities that they are not strongly motivated. Thus, they might be in a good position to detect both hyperactive/impulsive and inattentive symptoms. On the other hand, middle and high school teachers might spend few hours per week with students and might not detect well symptoms that do not disturb classes like inat- tentive and executive functioning symptoms. Regardless of their ability to accurately describe their symptoms, it is essential to fully include a child or young person in the assessment process. At the very least it is essential to ascertain their perspective about what it is like to be them. Important aspects include: yy How do they feel their symptoms impact on their lives? yy How is their self-esteem and quality of life? yy How do they feel about their sibling and peer relationships and relationships with parents and other key adults? ADHD AS AN HETEROGENEOUS DISORDER It is noteworthy that children with ADHD vary significantly from each other. ADHD, as other psychiatric disorders, is a highly heterogeneous disorder in res- pect to various aspects, such as symptom profiles, neuropsychological profiles, neurobiological and genetic features. One aspect of ADHD heterogeneity is related to its clinical presentation. Diag- nosis of mental disorders, according to diagnostic manuals, may be assigned from different combinations of criteria listed under the same disorder. In the case of ADHD, six symptoms in either of the two ADHD domains are required for an individual to meet diagnostic criteria during childhood or adolescence. Because the criteria are subdivided into symptom domains (inattention and hyperactivity/ impulsivity), it is possible that two individuals diagnosed with ADHD do not have the same group of symptoms. The classification of ADHD diagnosis into current presentation (predominantly inattentive, hyperactive-impulsive, and combined types) is an attempt to deal with the heterogeneity of clinical presentations. Even so, two individuals with the same ADHD current presentation might be similar in as few as three symptoms. This indicates the limited ability of the current clinical diagnostic criteria in defining homogeneous populations, which may be one reason why the field has not yet been successful in finding biological markers of ADHD. 14 Another facet of ADHD heterogeneity is neuropsychological heterogeneity. ADHD has been shown to be associated with various neuropsychological impair- ments. Studies have found that, on average, individuals with ADHD, compared to controls, have worse performance in various functions, including: inhibition,