Nitric oxide (NO) is a small free radical generated by a family of enzymes, the nitric oxide synthases (NOSs). Following injury to a tendon, NO is induced by all three isoforms of NOS and NOS activity is also upregulated in tendinopathy. In animal models when NOS activity is inhibited by competitive inhibitors of NOS, tendon healing is reduced. When additional NO is added, tendon healing is enhanced. In humans, in three randomised clinical trials, we have shown that NO delivered via a transdermal patch enhances the subjective and objective recovery of patients with tennis elbow, Achilles tendinosis and supraspinatus tendinosis.