As the use of free tissue transfer becomes more wide-spread, it is important for both the ablative surgeon and the reconstructive surgeon to understand the factors that contribute to successful revascularized tissue transfer. The purpose of this two part review is to provide a basic science overview of the problem of failed free tissue transfers. The first part will focus on the pathogenesis of thrombosis at the anastomotic site, and part two will discuss the pathogenesis of the no-reflow phenomenon. The pathophysiology and therapeutic interventions to prevent and treat anastomotic thrombosis and the no-reflow phenomenon will be discussed.