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Abstract: The Autism – Tics, AD/HD, and other Comorbidities inventory (A-TAC) is a comprehensive interview for evaluating problems related to autism spectrum disorders (ASD), tic disorders, attention-deficit/hyperactivity disorder (AD/HD), and common comorbid conditions in children and adolescents. A-TAC telephone interviews were administered to parents of 2,957 children aged nine- or twelve-years, representing one in each twin pair included in the population-based Child and Adolescent Twin Study in Sweden (CATSS). A total of 16.4 % were screen-positive for one or several of the targeted disorder, 1.3 % for ASD and 5.6 % for AD/HD. All types of problems were more common among boys, with the exception of those related to “eating habits”. They were all dimensionally/continuously distributed, highly inter-correlated, and overlapped across types. They aggregated in three ba-sic factors corresponding to externalizing/disruptiveness, socio-communicative problems, and compulsiveness. Population-based data on problems in children thus challenge current categorical diagnostic definitions, calling for dimen-sional and complementary models of problem descriptions