Non-alcoholic fatty liver disease (NAFLD) is considered to be a hepatic manifestation of metabolic syndrome. NAFLD has become an important public health issue because of its high prevalence. NAFLD consists of two clinicopathological entities: simple steatosis, which generally follows a benign non-progressive clinical course, and non-alcoholic steatohepatitis (NASH), which may progress to cirrhosis and hepatocellular carcinoma. The diagnosis of NAFLD is based on the following three criteria: non-alcoholic, detection of steatosis either by imaging or by histology, and appropriate exclusion of other liver diseases. Alcoholic liver disease can occur when daily alcohol consumption exceeds 20 g in women or 30 g in men. Thus, non-alcoholic indicates lower levels of these alcohol consumptions. However, there is still no clear consensus regarding the threshold alcohol consumption for defining non-alcoholic liver disease. Then, there is the strong recommendation for a change in the nomenclature, such as use of the term metabolic fatty liver and metabolic steatohepatitis. NASH has emerged as a clinicopathological entity, and even now, a liver biopsy remains the gold standard for making a definitive diagnosis. However, liver biopsy has several drawbacks. In general practice, NAFLD is a convenient-to-use term for the diagnosis and management of these patients, and serum biomarkers that indicate the severity of fibrosis serve as clinically useful tools for the identification of NAFLD in patients with bridging fibrosis or cirrhosis. In the future, improved understanding of the pathogenesis of NASH and new technologies may contribute to the diagnostic process and provide reliable, non-invasive alternatives to liver biopsy.