The final link in the transport of oxygen from the atmosphere to the cells is internal respiration or tissue oxygenation which involves the exchange of gases between capillaries and tissues. More than 90% of the body’s oxygen consumption is utilized by a single enzyme, cytochrome oxidase during the process of oxidative phosphorylation, which generates adenosine triphosphate (ATP). The main determinant of tissue oxygenation is a balance between oxygen delivery (DO2) and oxygen consumption (VO2). An imbalance between the two results in oxygen debt. This chapter highlights the steps in tissue oxygenation and provides an overview of tools for assessment of tissue hypoxia.