I n t h i s i s s u e...Child psychiatry is perhaps the most challenging subspe-cialty within our field. Clinicians and researchers mustconfront a variety of problems. Their patient is a movingtargetalways growing, developing, and changing.Generalizations made about a 5-year-old population (infact rarely studied) may not apply to a 10-year-old, andeven less so to a 15-year-old. Epidemiology, diagnosis,treatment, and neurobiology must use a developmentalperspective that recognizes the importance of vulnerabil-ity, change, and maturation during childhood and ado-lescence. Furthermore, many of the disorders that arecommon in young people have been defined anddescribed on the basis of their presentation in adults,despite the fact that we all recognize that children arenot miniature adults. In fact, this issue illustrates this par-ticular problem very well. All of the disorders discussed,apart from autism spectrum disorders, are defined anddescribed in the adult section of the American Psychi-atric Association Diagnostic and Statistical Manual ofMental Disorders (DSM-IV): bipolar disorder, unipolardepression, obsessive-compulsive disorder, sleep disor-ders, post-traumatic stress disorder, and anorexia ner-vosa. The need to decide how well these definitions anddescriptions apply to children and adolescents is a persis-tent theme in this issue, as is the need for more researchon this topic. Likewise, treatment protocols appropriatefor adults clearly cannot be easily extrapolated to childrenand adolescents. We know far too little about when andhow to intervene, as well as the benefits and risks ofearly intervention. One thing is certain: although childrenare not small adults, they will all become adults. The chal-lenge of reducing the global burden of mental illnessmust begin with high-quality diagnosis and treatment ofchildren and adolescents. This issue helps to address thatchallenge.The issue begins with a State of the art overview of theepidemiology of childhood disorders by KathleenMerikangas, Erin Nakamura, and Ronald Kessler (p 7). Asthey point out, these disorders contribute heavily to theglobal burden of disease in and of themselves. Beginningwith a description of basic epidemiologic concepts suchas incidence and prevalence, they go on to summarizeour current state of knowledge about the disorders thatare most common in children and adolescents. While theepidemiology of adult disorders has received substantialattention and acquired a solid database, data of compa-rable quality do not as yet exist for children and adoles-cents. Information about the epidemiology of mental ill-nesses in children under 6 is especially limited. Whatexisting data do demonstrate, however, is that classicchildhood disorders are relatively common (eg, attentiondeficit hyperactivity disorder [ADHD], conduct disorder),but that many adult disorders are also common (eg,depression, anxiety disorders). Furthermore, the costsassociated with these disorders are substantial, and yetthe services provided are insufficient. Only about half ofchildren and adolescents with mental disorders receivemental health services, even in developed nations suchas the US and the UK. Many experts are dissatisfied withthe current methods for classifying and diagnosing men-tal illnesses in young people, since they fail to account forthe high rates of comorbidity and the possibility thatsome disorders are characterized by overlapping dimen-sions of psychopathology. Two papers emphasizing Translational research echothemes expressed by Merikangas et al, particularly theimportance of conceptualizing childhood disorders withdimensional approaches. James Leckman, Michael Bloch,and Robert King (p 21) provide an elegant example ofthese approaches in the study of childhood obsessive-compulsive disorder (OCD), which affects approximately1% to 3% of children and adolescents. Although OCD isdescribed as a unitary nosological entity in DSM-IV andthe World Health Organization's ICD-10 Classification ofMental and Behavioral Disorders. Clinical descriptionsand diagnostic guidelines (ICD-10), multiple factor ana-lytic studies have suggested instead that it is comprisedof four factors or dimensions in both children and adults:forbidden thoughts, symmetry, cleaning, and hoarding.Leckman et al discuss the power of this dimensionalapproach, in combination with a developmental andneurobiological perspective. For example, imaging stud-ies suggest that different neural networks may beinvolved in the different dimensions, while differentgenes may be implicated for them at the molecular level.Taking a slightly different perspective, Joseph Buxbaum(p 35) focuses specifically on genetic etiologies of theautism spectrum disorders (ASD). In an interesting andcomprehensive summary of our knowledge to date, hebroadens the concept of ASD by describing its potentialoverlap with other rarer conditions such as Rett's syn-drome or 22q13 deletions. This article will be particular-ly useful for those readers who wish to become morefamiliar with cutting-edge genomics and genome-wide