A complete understanding of the mechanism of ammonia pyrolysis and oxidation in the full range of operating conditions displayed by industrial applications is one of the challenges of modern combustion kinetics. In this work, a wide-range investigation of the oxidation mechanism of ammonia was performed. Experimental campaigns were carried out in a jet-stirred reactor and a flow reactor under lean conditions (0.01 ≤ Φ ≤ 0.375), such to cover the full range of operating temperatures (500 K ≤ T ≤ 2000 K). Ammonia conversion and the formation of products and intermediates were analyzed. At the same time, the ammonia decomposition reaction, H-abstractions and the decomposition of the HNO intermediate were evaluated ab initio, and the related rates were included in a comprehensive kinetic model, developed according to a first-principles approach. Low-temperature reactor experiments highlighted a delayed reactivity of ammonia, in spite of the high amount of oxygen. A very slow increase in NH₃ consumption rate with temperature was observed, and a full reactant consumption was possible only ∼150–200 K after the reactivity onset. The use of flux analysis and sensitivity analysis allowed explaining this effect with the terminating effect of the H-abstraction on NH₃ by O₂, acting in the reverse direction because of the high amounts of HO₂. The central role of H₂NO was observed at low temperatures (T < 1200 K), and H-abstractions from it by HO₂, NO₂ and NH₂ were found to control reactivity, especially at higher pressures. On the other side, the formation of HNO intermediate via NH₂ + O = HNO + H and its decomposition were found to be crucial at higher temperatures, affecting both NO/N₂ ratio and flame propagation.