The knowledge of the lightning-induced nitrogen oxides (LNO_x) source is important for understanding and predicting the nitrogen oxides and ozone distributions in the troposphere and their trends, the oxidising capacity of the atmosphere, and the lifetime of trace gases destroyed by reactions with OH. This knowledge is further required for the assessment of other important NO_x sources, in particular from aviation, the stratosphere, and from surface sources, and for understanding the possible feedback between climate changes and lightning. This paper reviews more then 3 decades of research. The review includes laboratory studies as well as surface, airborne and satellite-based observations of lightning and of NO_x and related species in the atmosphere. Relevant data available from measurements in regions with strong LNO_x influence are identified, including recent observations at midlatitudes and over tropical continents where most lightning occurs. Various methods to model LNO_x at cloud scales or globally are described. Previous estimates are re-evaluated using the global annual mean flash frequency of 44±5 s^?1 reported from OTD satellite data. From the review, mainly of airborne measurements near thunderstorms and cloud-resolving models, we conclude that a "typical" thunderstorm flash produces 15 (2?40)×10²⁵ NO molecules per flash, equivalent to 250 mol NO_x or 3.5 kg of N mass per flash with uncertainty factor from 0.13 to 2.7. Mainly as a result of previous global model studies for various LNO_x parameterisations tested with related observations, the best estimate of the annual global LNO_x nitrogen mass source and its uncertainty range is (5±3) Tg a^?1 in this study. An accuracy of order 1 Tg a^?1 or 20%, as necessary in particular for understanding tropical tropospheric chemistry, is still a challenging goal.