Critical care clinicians must be knowledgeable about the anatomic, physiologic, and biochemical processes that are critical to the restoration of a functioning microvascular affecting organ perfusion. These basic physiologic processes critical to tissue perfusion and cellular oxygenation are presented in this issue on Monitoring Tissue Perfusion and Oxygenation. A working knowledge of oxygen delivery and oxygen consumption at the microvascular level will provide critical information needed for clinicians to continuously question the adequacy of tissue perfusion given our current lack of microvascular bedside monitoring.