The presence of multiple limitations for bleeding time has led to the development of automated assays assessing primary hemostasis. Platelet closure time measured with the Platelet Function Analyzer (PFA) is an innovative approach that explores primary hemostasis. Closure time has largely replaced bleeding time in the hospital setting due to its greater applicability. From a technical point of view, the test is equivalent in vitro to the traditional in vivo bleeding time. Thus, closure time is also known as in vitro bleeding time. This book is a review of the literature on platelet closure time. It is a detailed presentation of the PFA test and an update of the current knowledge of the use of this test in exploring primary hemostasis and monitoring the antiplatelet agents (APA) treatment. Finally, the manuscript gives a comparative performance analysis of this test and the classical tests studying primary hemostasis.