The formation of a hemostatic plug in response to vascular injury is a tightly regulated process that balances the prevention of blood loss with the avoidance of pathological thrombosis. This chapter examines the spatiotemporal dynamics of platelet activation and aggregation during hemostasis in vivo, emphasizing the graded nature of platelet activation states and the hierarchical organization of the hemostatic plug. The initiation, extension, and stabilization phases of plug formation are discussed, highlighting the critical roles of von Willebrand factor, thrombin, ADP, and thromboxane A2 in mediating platelet adhesion, recruitment, and cohesion in time and space. Drawing on advances in intravital imaging and computational modeling, the intricate interplay between molecular signaling pathways, platelet adhesion mechanisms, and the physical and biochemical microenvironment that governs the hemostatic response is explored. The chapter discusses how local flow dynamics, agonist diffusion, and localization of thrombin activity influence the kinetics and architecture of platelet plugs. It also examines how these processes vary across different vascular contexts, such as the microcirculation versus the macrocirculation. Approaching hemostasis from a systems perspective underscores opportunities for therapeutic intervention to distinguish physiological hemostasis from pathological thrombosis, offering insights into the development of targeted pro-hemostatic and antithrombotic strategies.