Non-alcoholic fatty liver disease is a chronic inflammation liver condition that is currently highly relevant because of its strong association with increasingly incident diseases such as obesity and type-2 diabetes mellitus. The primary purpose of this paper is to discuss the best part of current knowledge on the molecular mechanisms involved in hepatic steatosis development, the condition s initial stage, and on progression to steatohepatitis. Special attention has been paid to clinical and experimental obesity-related fatty liver. In the latter, the fa/fa rat is assessed, which constitutes an animal model for obesity with phenotype features similar to human obesity, including insulin resistance and dyslipemia. Hepatic steatosis is a complex, mainly metabolic condition where apparently non-compatible metabolic processes concur, in addition to oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction, and decreased expression of survival genes. Extrahepatic signals underlie the disorder, such as those arising from peripheral insulin resistance associated with an increase in adipose mass and systemic free fatty acids, together with intrahepatic signals leading to derangement of liver glycostatic and lipidostatic functions, as well as to greater vulnerability to other aggressions.