Cells use oxygen for aerobic metabolism localized in mitochondria but also for a variety of other oxygen dependent biochemical reactions. Thus all cells have a need for oxygen. There is a gradient of oxygen from the inspired air to the cells. Its steepness determines cellular oxygen supply. Determinants are the PO2 of inspired air, diffusion capacity of the alveolar wall, capacity of blood to bind oxygen, circulation of oxygen containing blood, distribution of blood to specific organs and to specific locations within the organ by the number of capillaries supplying cells, by the distance between cells and blood capillaries, and by the oxygen demand of the cell. Another determinant is the affinity of O2-binding and utilizing systems for oxygen. Thus, lung cells and cells in arterial blood are exposed to the highest PO2 whereas cells in bone marrow and exercising heart and skeletal muscle probably see the lowest level of oxygen. In this review various aspects of these systems will be discussed.