Nonalcoholic fatty liver disease (NAFLD) is a growing health issue, affecting 20-30% of adults, with higher prevalence in industrialized countries. It is characterized by fat accumulation in the liver in individuals who do not consume excessive alcohol. While most patients have simple steatosis, a minority develop inflammation and fibrosis, leading to non-alcoholic steatohepatitis (NASH), which increases the risk of liver cirrhosis and hepatocellular carcinoma. The 'two-hit' hypothesis, which proposed that insulin resistance and lipid accumulation were the main causes of NAFLD, is now considered outdated. The 'multiple-hit' hypothesis suggests that multiple factors, including insulin resistance, adipose tissue hormones, nutrition, gut microbiota, and genetic and epigenetic factors, contribute to the development of NAFLD and NASH. These factors interact in genetically predisposed individuals to cause liver damage. Insulin resistance leads to increased hepatic de novo lipogenesis and impaired adipose tissue lipolysis, contributing to fatty acid accumulation in the liver. This can lead to lipotoxicity, mitochondrial dysfunction, and endoplasmic reticulum stress, which activate inflammatory pathways. Inflammatory cytokines such as IL-6 and TNF-α are also involved in the progression of NAFLD. The gut microbiome plays a key role in NAFLD through the gut-liver axis, with altered microbiota contributing to increased intestinal permeability and the release of endotoxins. The gut microbiota also influences bile acid metabolism and the production of toxic compounds, which can lead to liver injury. Genetic factors, such as variations in the PNPLA3 gene, are also involved in the development and progression of NAFLD. Epigenetic factors, including DNA methylation and histone modifications, also play a role in the pathogenesis of NAFLD. Dietary factors, such as high fructose intake and consumption of sweetened beverages, contribute to the development of NAFLD and NASH. Conversely, a Mediterranean diet and moderate alcohol consumption may have protective effects. The gut microbiome is a key factor in the pathogenesis of NAFLD, with altered microbiota contributing to increased intestinal permeability and the release of endotoxins. The multiple-hit hypothesis provides a more accurate explanation of the complex pathogenesis of NAFLD, highlighting the role of various factors in the development and progression of the disease. Understanding these mechanisms is crucial for the development of new treatments and non-invasive markers for the diagnosis of NAFLD and NASH.Nonalcoholic fatty liver disease (NAFLD) is a growing health issue, affecting 20-30% of adults, with higher prevalence in industrialized countries. It is characterized by fat accumulation in the liver in individuals who do not consume excessive alcohol. While most patients have simple steatosis, a minority develop inflammation and fibrosis, leading to non-alcoholic steatohepatitis (NASH), which increases the risk of liver cirrhosis and hepatocellular carcinoma. The 'two-hit' hypothesis, which proposed that insulin resistance and lipid accumulation were the main causes of NAFLD, is now considered outdated. The 'multiple-hit' hypothesis suggests that multiple factors, including insulin resistance, adipose tissue hormones, nutrition, gut microbiota, and genetic and epigenetic factors, contribute to the development of NAFLD and NASH. These factors interact in genetically predisposed individuals to cause liver damage. Insulin resistance leads to increased hepatic de novo lipogenesis and impaired adipose tissue lipolysis, contributing to fatty acid accumulation in the liver. This can lead to lipotoxicity, mitochondrial dysfunction, and endoplasmic reticulum stress, which activate inflammatory pathways. Inflammatory cytokines such as IL-6 and TNF-α are also involved in the progression of NAFLD. The gut microbiome plays a key role in NAFLD through the gut-liver axis, with altered microbiota contributing to increased intestinal permeability and the release of endotoxins. The gut microbiota also influences bile acid metabolism and the production of toxic compounds, which can lead to liver injury. Genetic factors, such as variations in the PNPLA3 gene, are also involved in the development and progression of NAFLD. Epigenetic factors, including DNA methylation and histone modifications, also play a role in the pathogenesis of NAFLD. Dietary factors, such as high fructose intake and consumption of sweetened beverages, contribute to the development of NAFLD and NASH. Conversely, a Mediterranean diet and moderate alcohol consumption may have protective effects. The gut microbiome is a key factor in the pathogenesis of NAFLD, with altered microbiota contributing to increased intestinal permeability and the release of endotoxins. The multiple-hit hypothesis provides a more accurate explanation of the complex pathogenesis of NAFLD, highlighting the role of various factors in the development and progression of the disease. Understanding these mechanisms is crucial for the development of new treatments and non-invasive markers for the diagnosis of NAFLD and NASH.