Flexor tenosynovitis, infection of the flexor tendon sheath, is a time-critical surgical emergency that is associated with long term sequalae if the diagnosis is missed. In 1912, Dr Kanavel first described the three cardinal signs of flexor tenosynovitis: extreme pain over the course of flexor sheath, fixed flexion of affected finger and pain on extension. The widely accepted fourth sign of fusiform swelling was described in his initial publication, but was not included in his primary three classic signs. The Kanavel signs have since been used as the diagnostic tool for flexor tenosynovitis, even though there are no systemic studies that validate it. There is no superiority of any signs that is more predictive of the diagnosis. Patients should not be excluded from a diagnosis of flexor tenosynovitis even if they fail to meet the full criteria of Kanavel’s sign. Complications of delayed management are detrimental including loss of hand function, deformity and digit amputation. Clinicians must be vigilant in the suspicion for the presence of flexor tenosynovitis.