This chapter brings a detailed review on the cell components in plasma, and their associated functions like haemostasis and inflammation, as well as the mechanism of platelet activation. The issue is described in a didactic and comprehensive format. The authors also include practical applications of PRP in orthopaedic diseases such as tendinopathies, chondral lesions, osteonecrosis and ACL lesions. The association of PRP, growth factors and mesenchymal stem cells is pointed out as a future treatment for orthopaedic lesions.