Cancer is a multifactorial disease that most commonly affects people ≥50 years of age. However, evidence indicates that the incidence of cancers of various organs (including those of the breast, colorectum, endometrium, oesophagus, extrahepatic bile duct, gallbladder, head and neck, kidney, liver, bone marrow (multiple myeloma (MM)), pancreas, prostate, stomach and thyroid) has been rising in adults <50 years of age in many parts of the world 1-13 . This trend is also observed in analyses using Global Cancer Observatory (GLOBOCAN; https://gco.iarc.fr/) data (Fig. 1 provides data on selected countries; more comprehensive data are provided in Supplementary Table 1). We herein use the term 'early-onset' to describe cancers diagnosed in adults <50 years of age, and a contrasting term, 'later-onset' , for cancers diagnosed in those ≥50 years of age. Cancers diagnosed during childhood and adolescence (<20 years of age) are out of the scope of this Review.The rise of early-onset cancer has considerable personal, societal and economic implications. Survivors of early-onset cancers have a higher risk of long-term health problems such as infertility, cardiovascular disease and secondary cancers [14][15][16] . Owing to this increasing cancer burden among young adults, which might be referred to as the 'early-onset cancer epidemic' , the US National Cancer Institute listed this phenomenon as a research priority in one of its 'Provocative Questions' for 2020-2021 (reF. 17).Differences in epidemiology and clinical, pathological, and molecular characteristics clearly exist between early-onset and later-onset cancers, although these features are not likely to change dramatically at exactly 50 years of age 18 . Furthermore, early-onset cancer in any given organ is not a homogeneous entity but rather encompasses a variable range of clinical and pathological features 18,19 . We acknowledge the limitations of applying a dichotomy at 50 years of age, although we selected this cut-off point to enable consistent collection and interpretation of current evidence on early-onset cancers. In reality, we also need to consider heterogeneity within Is early-onset cancer an emerging global epidemic? Current evidence and future implications

RR, relative risk; ref, reference; GDM, gestational diabetes mellitus. Data are n/N (%) unless otherwise specified. * 95% confidence interval. † 99% confidence interval. ‡ Among pregnancies. § Multivariable model adjusted for age, year of pregnancy, and year of pregnancy interaction term. ║ Multivariable model additionally adjusted for race, age at menarche, menstrual cycle length between ages 18 and 22 years, BMI at age 18 years, smoking status, alcohol intake, parity, history of infertility, multiple gestation (preeclampsia or gestational hypertension, preterm birth) and preterm birth (low birth weight). ¶ Among births.