any studies in the past decade have elaborated the emerging functions of platelets beyond hemostasis and thrombosis. Molecular and functional studies have shown diverse roles of platelets in vascular integrity and remodeling, immunoregulation and tissue regeneration (Fig. 1). Although the function of platelets in the development and progression of cardiovascular diseases is well established, platelets were also demonstrated to be critical players in the pathophysiology of cancer, inflammatory diseases and infections such as that with severe acute respiratory syndrome coronavirus (SARS-CoV)-2, giving rise to COVID-19. Investigations of platelets and their heterogeneous states, platelet releasates and other target cells in various diseases have provided important mechanistic insights for diagnosis, prognosis and therapeutics. In this Review, we discuss the pathophysiological role of platelets and the latest developments in basic and clinical platelet methodologies. We also provide new insights into the development of guidelines for platelet investigation in which knowledge of underlying mechanisms might be important for diagnosis.