Ask any mountain climber or deep-sea diver: when it comes to oxygen deprivation, timing is everything. The body is not only affected by how long it goes without oxygen, but also by the time it takes for its cells to react to this shortage of oxygen, also known as hypoxia. Both rapid and delayed responses have a profound impact on normal physiological function, and on the progression of diseases such as pulmonary hypertension, sleep apnea, cancer, and neurodegenerative disorders. This volume explores the transcriptional and pathophysiological responses to hypoxia and of the sensing mechanisms responsible for detection of oxygen level changes in the body.