Prevalence estimates of the attention-deficit hyperactivity disorder (ADHD) and the rate of persistence of symptoms across the lifespan are heterogeneous, raising questions about the validity of the diagnosis. This review aims to discuss potential reasons for variability in ADHD prevalence estimates and rates of symptom persistence, as well as to present ADHD prevalence rates during the lifespan. The best available estimates of ADHD prevalence are around 5.29% for children and adolescents and 4.4% in adulthood. Estimates of ADHD prevalence and rate of symptom persistence over time seem to be highly affected by methodological characteristics of the studies. The review of ADHD epidemiology highlights the need for standardizing study methodologies to make findings comparable. Even so, epidemiological cross-national data seem to support the validity of ADHD.