Subthreshold disorders, conditions with relevant psychiatric symptoms which do not meet the full criteria of a disorder according to the prevailing classification systems, have received increased attention recently. The current paper aims to present a systematic review of subthreshold attention-deficit/hyperactivity disorder (ADHD) in children and adolescents. Searching five computerised databases (Ovid MEDLINE, Psychinfo, PubMed, Scopus, Web of Science) with two categories of search terms [(1) subclinical; subsyndromal; subthreshold (2) ADHD] the authors examined the prevalence of subthreshold ADHD among children and adolescents, the comorbidity of subthreshold ADHD and whether there was already any impact of subthreshold ADHD on functioning. Before these questions were answered, the included articles were examined to see what kinds of definitions of child and adolescent subthreshold ADHD are used and what kinds of assessments are used for measuring subthreshold ADHD among children and adolescents. The results of the 18 articles included show that different definitions of subthreshold ADHD in children and adolescents exist, a large variety of instruments are used, the prevalence rate of subthreshold ADHD is wide-ranging (0.8–23.1 %), the comorbidity of subthreshold ADHD is high and there are several areas where subthreshold ADHD has a meaningful impact on functioning. All these suggest that focusing on subthreshold ADHD can be important in preventative interventions. The results of this systematic review support the dimensional approach of ADHD. Further research on uniform criteria of subthreshold ADHD is needed to support the inclusion of this condition in classification systems.