Nonalcoholic fatty liver disease (NAFLD) is the most common cause of liver dysfunction in the Western world and is increasing owing to its close association with obesity and insulin resistance. NAFLD represents a spectrum of liver disease that, in a minority of patients, can lead to progressive nonalcoholic steatohepatitis (NASH), fibrosis, and ultimately hepatocellular carcinoma and liver failure. NAFLD is a complex trait resulting from the interaction between environmental exposure and a susceptible polygenic background and comprising multiple independent modifiers of risk, such as the microbiome. The molecular mechanisms that combine to define the transition to NASH and progressive disease are complex, and consequently, no pharmacological therapy currently exists to treat NASH. A better understanding of the pathogenesis of NAFLD is critical if new treatments are to be discovered.