Oxygen is biologically vital element sustaining life. The tissue oxygen delivery is therefore precisely regulated. The degree of tissue oxygenation is estimated by measurement of oxygen blood level. The lack of oxygen on cellular and tissue level can lead to organ failure and life-threatening condition. Important adaptive processes are activated during the sublethal hypoxia with goal to preserve cellular and tissue functions. Inadequate effort to correct hypoxia can cause either disturbance of the adaptation or undesirable tissue hyperoxia. This fact is taken into account in two currently proposed concepts: (1) precise control of arterial oxemia and (2) permissive hypoxemia. Recent literature supports rather restrictive strategy of oxygen therapy in critical care.