Many platelet function assays have been developed over the last century in an attempt to capture the natural ability of platelets to form aggregates in response to vascular injury. From physiological assays, such as the bleeding time, to diagnostic assays used within specialized hematology departments, to point-of-care assays that are intended as clinical decision aids on wards and in operating rooms, and finally to high-throughput deep phenotyping assays intended for precision medicine, platelet function assays have become increasingly commonplace in many settings. This chapter presents an overview of the most commonly used platelet function assays and discusses important variables to take into account when performing platelet function testing, ranging from pre-analytical issues to the clinical utility of individual tests.