This article reviews the impact of ultra-processed foods (UPFs) on the human gut microbiome and the variations in classification methods, particularly the NOVA system. UPFs, which are industrially processed and often energy-dense with low nutrients, are prevalent in the Western diet and have been linked to non-communicable diseases like obesity and cardiovascular disease. The review highlights that UPF consumption is associated with negative health outcomes, including alterations in the gut microbiome. Despite the growing body of research, there is no consensus on UPF classification, making it challenging to compare studies. The NOVA classification system, which categorizes foods based on processing levels, is widely used but faces challenges in application due to ambiguities and varying methodologies. The article also discusses the effects of UPFs on the gut microbiome, noting changes in microbial diversity and specific bacterial groups. It emphasizes the need for more detailed studies on the mechanisms by which UPFs affect health, particularly through the microbiome and metabolome. The review concludes that future research should focus on refining UPF classification and understanding their health impacts to inform public health policies.This article reviews the impact of ultra-processed foods (UPFs) on the human gut microbiome and the variations in classification methods, particularly the NOVA system. UPFs, which are industrially processed and often energy-dense with low nutrients, are prevalent in the Western diet and have been linked to non-communicable diseases like obesity and cardiovascular disease. The review highlights that UPF consumption is associated with negative health outcomes, including alterations in the gut microbiome. Despite the growing body of research, there is no consensus on UPF classification, making it challenging to compare studies. The NOVA classification system, which categorizes foods based on processing levels, is widely used but faces challenges in application due to ambiguities and varying methodologies. The article also discusses the effects of UPFs on the gut microbiome, noting changes in microbial diversity and specific bacterial groups. It emphasizes the need for more detailed studies on the mechanisms by which UPFs affect health, particularly through the microbiome and metabolome. The review concludes that future research should focus on refining UPF classification and understanding their health impacts to inform public health policies.