Activation of platelets plays a central role in hemostasis as well as in various thromboembolic diseases like myocardial infarction or stroke. Most platelet activating stimuli function through receptors which couple to heterotrimeric G proteins of the G_i, G_q and G₁₂ families. Recent studies have elucidated the roles of individual G proteins in the regulation of platelet functions like shape change, aggregation and granule secretion. The signaling pathways mediated by heterotrimeric G proteins operate synergistically to induce a full activation of platelets. This review summarizes recent progress in the understanding of upstream regulation of platelet activation through G protein-coupled receptors.