Over the last few years, the paradigm in hepatology has changed from focusing on a single liver disease to considering concurrent diseases, in particular obesity and related metabolic factors. Obesity has reached epidemic proportions globally and is associated with insulin resistance, steatosis and a low-grade systemic inflammatory state. These metabolic factors have a synergistic role in the natural history and treatment outcomes related to chronic liver disease. This is characterized best in chronic hepatitis C where steatosis and insulin resistance are caused by viral and metabolic effects. Non-alcoholic fatty liver disease and related metabolic abnormalities also exacerbate other diseases, such as alcoholic liver disease and haemochromatosis. In addition, there is growing evidence linking obesity and type 2 diabetes with hepatocellular carcinoma in subjects with chronic viral hepatitis. The pathogenesis of co-morbid disease may be related to increased oxidative stress, inflammatory injury and cell death, along with altered hepatocyte regeneration and repair. Hyperinsulinaemia and other metabolic factors may also have a direct role in the progression of liver injury. Data indicate that weight reduction improves steatosis and inflammation in patients with chronic hepatitis C. This has important clinical and therapeutic implications and suggests that obesity should be actively addressed in the management of patients with other chronic liver diseases.