Measuring molecular oxygen levels in vivo has been the cornerstone of understanding the effects of hypoxia in normal tissues and malignant tumors. Here we discuss the advances in a variety of partial pressure of oxygen (
) measurements and imaging techniques and relevant oxygen thresholds. A focus on electron paramagnetic resonance (EPR) imaging shows the validation of treating hypoxic tumours with a threshold of 
≤ 10 Torr, and demonstrates utility for in vivo oxygen imaging, as well as its current and future role in cancer studies.