Non-alcoholic steatohepatitis is a chronic disease that occurs in persons without significative consumption of alcohol, characterized by macrovesicular steatosis, mixed inflammatory infiltrate, and diverse degrees of fibrosis. It can progress to cirrhosis and its evolution to hepatocellular carcinoma has been described. It principally occurs in patients with obesity, diabetes mellitus, and hyperlipidemia, and is at present considered a manifestation of metabolic syndrome with insulin resistance. In pathogenesis, diverse factors, fundamentally insulin resistance as a mechanism that determines hepatic steatosis, have been described. Later, alteration of signalling cascades, oxidative stress, and other mechanisms occur that lead to inflammation, necrosis, and finally to hepatic fibrosis, the details of which will be described in this review.