Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of fat in the liver with or without inflammation, fibrosis and cirrhosis, in the absence of substantial alcohol consumption. In this thesis, several potential causes of NAFLD from across the life course, have been investigated, as well as associations of NAFLD with major clinical outcomes. This was done using data from three population based studies: the British Women's Heart and Health Study, a prospective cohort of 4,286 British women ages 60-79; the Caerphilly Prospective Study, a cohort of 2,512 Welsh men ages 40-59; the National Health and Nutrition Examination Survey - an ongoing survey of a representative sample of the US population; and a randomized controlled trial, the Dietary Interventions in Type 2 Obese Diabetic Patients in the Community. Alanine aminotrasferase (ALT) and gamma-glutamyltransferase (GGT) were used as biomarkers of NAFLD and liver fat accumulation.