This book investigates various aspects of the phenomenon of atmospheric oxygen depletion, and summarizes our current knowledge of atmospheric oxygen. The authors address the currently available records indicating that atmospheric oxygen is undergoing nonlinear decline. The consensus in the research community is that the main reason for this decline is the combustion of fossil fuels (which is growing nonlinearly). However, the short length of the currently available records, alongside the technological and social uncertainties that may risk speed up oxygen depletion, raise questions about the time scale of the decline, in particular, when oxygen depletion may cause physiological impact on humans and animals. The authors analyze this uncertainty to postulate the near term impact of depleted atmospheric oxygen, and perform multidisciplinary complex modelling of oxygen with long-term horizon estimations.