The ability of platelets to form stable adhesion contacts with other activated platelets (platelet cohesion or aggregation) at sites of vascular injury is essential for hemostasis and thrombosis. In this study, we have examined the mechanisms regulating cytosolic calcium flux during the development of platelet–platelet adhesion contacts under the influence of flow. An examination of platelet calcium flux during platelet aggregate formation in vitro demonstrated a key role for intercellular calcium communication (ICC) in regulating the recruitment of translocating platelets into developing aggregates. We demonstrate that ICC is primarily mediated by a signaling mechanism operating between integrin α_IIbβ₃ and the recently cloned ADP purinergic receptor P2Y₁₂. Furthermore, we demonstrate that the efficiency by which calcium signals are propagated within platelet aggregates plays an important role in dictating the rate and extent of thrombus growth.