Summary: The technique of vascular delay has been used by plastic surgeons for nearly 500 years and has proven useful for reliably transferring tissue and allowing for a greater volume of tissue to be reliably harvested. Delay procedures are an essential plastic surgical tool for a variety of aesthetic and reconstructive procedures. Despite the widespread use of vascular delay procedures, the mechanism by which this phenomenon occurs remains unclear. A number of groups have exhaustively examined microvascular changes that occur during vascular delay. Theories have been proposed ranging from the dilation of choke vessels to changes in metabolism and new blood vessel formation. Inherent in these theories is the concept that ischemia is able to act as the primary stimulus for vascular changes. The purpose of this review is to revisit the theories proposed to underlie the delay phenomenon in light of recent advances in vascular biology. In particular, the participation of bone marrow–derived endothelial progenitor cells in the delay phenomenon is explored. Greater understanding of the role these cells play in new blood vessel formation will be of considerable clinical benefit to high-risk patients in future applications of delay procedures.