Summary The prevalence of nonalcoholic fatty liver disease (NAFLD) and the more severe and inflammatory type, nonalcoholic steatohepatitis (NASH), is increasing rapidly. Especially in high‐risk patients, that is those with obesity, metabolic syndrome, and type 2 diabetes mellitus, the prevalence of NAFLD can be as high as 80% while NASH may be present in 20% of these subjects. With the worldwide increase of obesity, it is most likely that these numbers will rise. Since advanced stages of NAFLD and NASH are strongly associated with morbidity and mortality—in particular, cardiovascular disease, liver cirrhosis, and hepatocellular carcinoma—it is of great importance to identify subjects at risk. A great variety of noninvasive tests has been published to diagnose NAFLD and NASH, especially using blood‐ and imaging‐based tests. Liver biopsy remains the gold standard for NAFLD/NASH. This review aims to summarize the different mechanisms leading to NASH and liver fibrosis, the different noninvasive liver tests to diagnose and evaluate patients with severe obesity.