In this volume, the authors will describe the mechanisms of hypoxia and angiogenesis from a systems perspective. Hypoxia - defined as low oxygen - is quantitative by nature, and species dependent. Humans evolved to function in the Earth's atmosphere of 21% O2. At even 1% lower atmospheric oxygen levels, the body responds with a host of mechanisms unactivated in normal oxygen levels. Oxygen levels in cells can be altered by multiple means, including changes in inspired oxygen, lack of oxygen-carrying hemoglobin from the blood, increased cellular oxygen demand and changes in the cell growth rates.