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Background: Agni, usually translated as digestive fire, is the central concept of Ayurvedic physiology and pathology. It is held responsible for digestion, metabolism, strength and immunity, and disorders of agni are regarded as the root of many diseases. Objective: To review the classical description of agni, its types, functions and disorders, and the concept of ama, and to discuss current research relating agni to metabolism, the gut microbiota and metabolic disease. Methods: Classical Ayurvedic texts were read in translation. PubMed was searched in November 2026 for papers on agni, ama, digestion and metabolism in Ayurveda. Results: The classical texts describe thirteen forms of agni: one jatharagni, five bhutagnis and seven dhatvagnis, and four states of jatharagni (sama, vishama, tikshna and manda). Impaired agni produces ama, which obstructs the srotas and causes disease. Recent work proposes links between agni and metabolic regulation, gut microbiota and metabolic heterogeneity in prameha and obesity. Conclusion: Agni is a functional concept covering digestion and metabolism at several levels. Its use in research requires clear operational definitions and measures of agni status, and caution against simple identification with a single enzyme system or organ.