Intraindividual variability (IIV) - reflecting short-term (within-session), within-person fluctuations in behavioral performance - and, specifically, reaction time (RT) variability, is strongly linked with attention-deficit hyperactivity disorder (ADHD) both at the phenotypic and genetic levels. Phenotypic case-control comparisons show a consistent and robust association between ADHD and RT variability across a broad range of cognitive tasks, samples, and age ranges (from childhood to adulthood). The association does not appear to be a nonspecific effect mediated by lower general cognitive ability. The finding from quantitative genetic studies of the shared genetic etiology between ADHD and RT variability is similarly robust, replicating across tasks, samples, and definitions of ADHD. Molecular genetic studies have produced intriguing initial findings: increasing sample sizes and replications across datasets remain priorities for future efforts. While the field has come a long way from considering increased RT variability in ADHD as the "noise" or "error" that we need to reduce in our data, the investigation of the causal pathways is only beginning. The neural basis of IIV is being investigated, with initial data pointing to a crucial role of fronto-striatal systems in controlling behavioral consistency. Several theories have been put forward to account for the observed IIV in ADHD, including accounts of arousal regulation, temporal processing and the "default-mode network." For the wider implications of the IIV phenomenon to be fully realized, we need to learn further about the underlying processes, their developmental context, and about shared and unique causal pathways across disorders where high RT variability is observed.