Not all critically ill patients in an intensive care unit (ICU) require supplemental oxygen. The need is determined by a combination of clinical and laboratory parameters. However, in patients admitted to the ICU, reliance on clinical features alone can be hazardous, and clinical information therefore needs to be interpreted in context of more objective measures. Similarly, the mode of oxygen delivery in an ICU patient depends on the critical nature of the illness. This chapter discusses the assessment of hypoxemia in the ICU setting and methods of oxygen delivery for patients admitted to the ICU.