This article discusses technical and practical aspects of the limiting oxygen concentration (LOC) including a detailed appraisal of possible sources of measurement error. LOC values obtained in the classical 5-cm diameter, vertical tube apparatus have been described as nonconservative owing to flame quenching. However, much of the difference between classical LOC values and those obtained in closed vessels using ASTM E2079 is due to the change in the definition of ignition. Formerly defined as propagation of a visible flame through a vertical tube at least 1.5 m tall, which in a closed tube would typically generate an overpressure in excess of 100%, ignition is now defined as an overpressure that exceeds 7%. The small pressure rise criterion tends to underestimate the LOC in smaller test vessels, especially for tests made at elevated pressures and temperatures. More use should be made of ASTM E2079's flexibility regarding the use of larger pressure rise criteria. Having lost sight of the flame, it is important not to lose sight of the objective, namely to select test criteria that correctly predict the flammable limits pertaining to large volumes of gas mixture. Additional reference quality LOC data are needed both to improve the definition of ignition with respect to test vessel volume and to investigate the accuracy of LOC estimation techniques. Any large-scale reference quality test program should investigate the effect of low levels of forced turbulence on measured LOC values. The NFPA Explosion Protection Systems Technical Committee recently supported the development of two Tentative Interim Amendments (TIAs) to NFPA 69 Standard on Explosion Prevention Systems. The first TIA resolves issues with the table of LOC values for gases, which had been modified and over-corrected in the 2008-2014 editions. The second TIA introduces a simple method for estimating the LOCs of single gases and gas mixtures based on the equation LOC=LFL x S, where LFL is the lower flammable limit and S the molar stoichiometric ratio of oxygen to fuel. An Appendix describes the two TIA recommendations while a second Appendix describes how the estimating methods have been incorporated into ASTM's CHETAH Program. (c) 2016 American Institute of Chemical Engineers