Nonalcoholic fatty liver disease (NAFLD) is a significant public health problem. An estimated 30% of adults and 10% of children are affected, making NAFLD the most common chronic liver disease in the United States. However, there is an incomplete understanding of this disease. The association between NAFLD and quality of life (QOL) remains unclear. These data are important to estimate the burden of illness in NAFLD. Also unclear is the association between exercise and NAFLD severity. These data are important given that exercise is recommended as primary treatment for NAFLD. Individuals with biopsy- proven NAFLD, enrolled in the NASH CRN, were included in these analyses. Chapter 1 reports QOL in adults with NAFLD. Chapter 2 reports QOL in children with NAFLD. Chapter 3 reports the association between exercise intensity and NAFLD histological severity. Both adults (n=713) and children (n=240) with NAFLD had a significant decrement in QOL, especially physical health, compared to reference populations without NAFLD. Increased NAFLD severity was associated with poorer physical health in adults while QOL did not significantly differ based on severity in children. In children, symptoms accounted for almost half of the variance in QOL scores, especially fatigue, trouble sleeping, and sadness. A total of 609 individuals with NAFLD were included in the study of exercise intensity and histological severity. Participants who met the U.S. DHHS/USDA physical activity recommendation for vigorous physical activity (>̲75 min/ week) had a decreased odds of steatohepatitis, and individuals who met the àdditional health benefits' recommendation for vigorous physical activity (>̲150 min/ week) had a decreased odds of advanced fibrosis. No significant associations were found between moderate- intensity physical activity and histological severity. Adults and children with NAFLD have a significant decrement in QOL. In children, symptoms are a major determinant of impaired QOL. Strategies to improve QOL, especially physical health, should be a focus for clinicians treating patients with NAFLD. These results also suggest that current exercise treatment recommendations for NAFLD may be insufficient. Collectively these studies expand the understanding of NAFLD and highlight avenues for further research. Implications of the results are discussed in greater detail within each of the respective chapters.