The platelet has traditionally been associated with disorders of the cardiovascular system; a well-recognized cell type actively involved in the maintenance of haemostasis and the initiation of repair following tissue injury. It has been accepted that the primary function of platelets is their adhesion to the endothelium or to other components at sites of the injured vessel wall in the initiation of haemostasis. However, it has been suggested that the fundamental physiological role of the platelet within the mammalian circulation is in the defence of the host against invasion by foreign organisms. Studies from several groups suggest an important role of the platelet in allergic processes and immunological mechanisms. In this review, we have summarized the origin, physiology, activation and function of the platelet, in addition to both experimental and clinical evidence implicating the involvement of this cell type in certain human lung diseases.