In the late 1990s, the antiplatelet agent clopidogrel, a P2Y12 inhibitor, was introduced into clinical practice. Concurrently, several new methods for assessing platelet function emerged, such as the PFA-100 in 1995, marking the beginning of a sustained expansion in platelet function testing. It soon became apparent that patient responses to clopidogrel varied significantly, with some individuals exhibiting high on-treatment platelet reactivity. This variability prompted discussions around the utility of platelet function testing to tailor antiplatelet therapy. Additionally, such testing was proposed for patients preparing for cardiac surgery to better manage the balance between thrombotic risk before surgery and bleeding risk during the perioperative period. This chapter explores widely used platelet function tests in these contexts, particularly those considered point-of-care (POC) or requiring minimal laboratory processing. It also reviews recent guidelines and clinical trial evidence, building on a previously published chapter, regarding the role and effectiveness of platelet function testing in these clinical scenarios.