Epidemiological studies have shown that neurodevelopmental disorders (NDDs), such as autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD) are more prevalent in the general childhood population, compared to cases that are formally diagnosed in clinical cohorts. This suggests that many children and youths have NDD which are never diagnosed clinically, causing impairments in some domains of their daily life. There is increasing recognition of the concept of a "subthreshold" condition, sometimes used to describe the presence of potentially impairing variations in the neurodevelopmental profile that do not meet criteria for a diagnosis. The aim of this narrative review is to appraise the published literature about common themes regarding subthreshold conditions in relation to autism and ADHD, identifying any practical lessons that may be applicable to frontline neurodevelopmental clinicians. We searched electronic databases including PMC and PubMed using various combinations of keywords, including "Subthreshold", "subclinical", "neurodevelopmental", "childhood", "ADHD" and "ASD". The identified themes include definitions, prevalence, assessment tools, lifetime impairments, NDD classification models, management, raising public awareness, and future research directions. The authors propose that a "subthreshold condition" should be recorded when NDDs do not meet current diagnostic criteria if there is evidence of significant, persisting impairment in at least one setting.