Due to the rising prevalence of obesity, non-alcoholic fatty liver disease (NAFLD) has become the leading cause of chronic liver disease worldwide. NAFLD refers to a disease spectrum that encompasses steatosis and non-alcoholic steatohepatitis (NASH). NASH, which is the inflammatory subtype of NAFLD, has a clear potential of progression to fibrosis and cirrhosis, and can be associated with the need for transplantation. Identification of NAFLD and NASH is important in order to prevent disease worsening and to provide adequate tools for counteracting the causal factors. Risk factors associated with NAFLD and NASH include clinical comorbidities such as the metabolic syndrome, which is more relevant than high body mass index. Further recently identified characteristics, such as dietary composition, intestinal dysbiosis, genetic predisposition, altered brown adipose tissue, muscle alterations, circadian clock disruption or environmental chemicals, are presented in this review.