This thesis, presented as a thesis incorporating publications, examines the epidemiology, aetiology and genetics of myopia. It comprises work utilising two twin cohorts, Twins Early Development Study (TEDS) and TwinsUK, and collaborative work with the European Eye Epidemiology consortium (E3), EUREYE Study and Consortium for Refractive Error and Myopia (CREAM). The prevalence of refractive error across Europe was defined using a large, metaanalysis (E3). Myopia prevalence was higher in younger generations (47% in 25-29 year-olds), with clear evidence of higher rates for those born in the latter half of the twentieth century. There was a strong association between myopia and education - in E3 myopia prevalence was almost double in those with higher compared to primary education (43% vs. 24% in 50-54 year-olds). Similarly in EUREYE those going onto higher education had twice the odds of myopia (OR 2.08). In TEDS educational attainment and cognitive ability were associated with myopia only in adolescence; significant factors maintained in life-course models were maternal education (OR 1.33), fertility treatment (OR 0.63), summer birth (OR 1.93), and video games (OR 1.03). Time outdoors was replicated to be inversely associated with myopia in EUREYE, particularly in younger life (OR 0.81), but no evidence for mediation by vitamin D was observed. The complex interplay between environmental and genetic factors was examined in TEDS. Evidence of a shared genetic risk for myopia and higher intelligence was demonstrated using twin modeling (78% of phenotypic correlation explained by genetic factors) and polygenic risk scores. A suggestive interaction between genetic variants for myopia and near work was identified for five loci in CREAM. Myopia is becoming more common in Europe. Replicated and novel associations with myopia reflect societal trends and the complex, powerful interplay between genetics and environment. This has implications for future risk prediction and therapeutic interventions.