Ultra-processed foods (UPFs) is a food category within the NOVA system. The key feature of UPFs are foods that have been highly processed and contain various additives, especially those that are industrially produced. It is claimed that UPFs are inherently unhealthy. The classification system is highly controversial. This paper critically evaluates the evidence. In stark contrast to conventional systems for food classification, the NOVA system disregards the nutritional values of foods. As a result, many foods generally considered to be healthy are included as UPFs, whereas many unhealthy foods are excluded. Epidemiological studies, mainly prospective cohort studies, have consistently reported an association between the intake of UPFs and risk of obesity, cardiovascular disease (CVD), type 2 diabetes, common mental disorders (especially depression), and all-cause mortality. A similar association has been reported for cancer and hypertension, but the supporting evidence is weaker. The most plausible explanation for this is that the associations are largely due to a limited number of unhealthy foods, such as processed meat and sugar-sweetened beverages. Studies of the relationship between UPFs and obesity present a different picture. There is much evidence that suggests that UPFs play a major causal role in obesity. The high contents of fat, sugar, carbohydrates, and sodium commonly present in UPFs makes these foods hyperpalatable. In addition, UPFs typically have a high energy density. As a result of these two features of UPFs, most people consume an excessive energy intake when presented with UPFs. Because UPFs include a wide range of foods, many of which are healthy, it is likely that while many UPFs are obesogenic, many others are not.