Platelet activation is a critical event in haemostasis that relies on the ability of the platelet to correctly identify and respond to blood vessel damage. The cascade of paracrine signalling events initiated by platelet activation would trigger system-wide thrombosis were they not constrained by negative feedback and inhibitory processes. Dysregulation of these processes in conditions, such as diabetes, can lead to predisposition to thrombotic events. Equally, pharmacological manipulation of the cellular machinery that mediates negative feedback is important for current and novel antiplatelet therapies. Some negative regulators act as gatekeepers that modulate key mediators of activation and provide barriers that must be deactivated to permit full activation and stable thrombus formation. Other negative regulators come into play once platelets are activated and provide restraining, negative feedback for activatory pathways. Many endogenous inhibitors of platelet activation have been identified but not fully characterised, and so, questions remain regarding the mechanisms that underlie the effects on platelet activity following their activation, inhibition or genetic disruption. In this chapter, we have described endogenous inhibitors of platelet function that provide inhibition or negative regulation to the processes that underpin platelet activation and thrombosis.