Peripheral nerve injury is nerve damage caused by trauma (laceration, blunt, gunshot, avulsion), surgical complication, compression (compartment syndrome, tourniquet), traction (brachial plexus) or iatrogenic causes. The peripheral nervous system consists of nerves conveying motor, sensory and autonomic functions; post-injury weakness (motor deficit), numbness/paresthesia (sensory deficit) and vasomotor changes (autonomic dysfunction) develop. Nerve injury severity is classified by Seddon classification (3 degrees) or Sunderland classification (5 degrees). Seddon classification: (1) Neurapraxia - myelin sheath damage, axonal continuity preserved, complete recovery in 6-12 weeks; (2) Axonotmesis - axon severed but endoneurium (nerve sheath) intact, Wallerian degeneration occurs, regeneration slow (1 mm/day = 1 inch/month), intermediate prognosis; (3) Neurotmesis - complete nerve severance, all layers including endoneurium damaged, no spontaneous recovery, surgery required. Sunderland classification: Grade I (neurapraxia), Grade II (axon severed, endoneurium intact), Grade III (axon + endoneurium severed, perineurium intact), Grade IV (axon + endoneurium + perineurium severed, epineurium intact - neuroma-in-continuity), Grade V (complete transection - neurotmesis). Prevalence of nerve injury in trauma patients is 1-3%; upper extremity (median, ulnar, radial nerves) affected more frequently than lower extremity. Most common mechanism is laceration (30-40%), gunshot (20-30%) and blunt trauma (15-20%). Brachial plexus injury (avulsion) seen in motorcycle accidents. Diagnosis made by clinical examination (motor/sensory/autonomic tests), electrophysiological evaluation (EMG/NCS - detects Wallerian degeneration 2-3 weeks post-injury, fibrillation potentials), ultrasonography (high-resolution US - nerve continuity, neuroma, scar tissue) and MR neurography (plexus avulsion, nerve anatomy). Treatment approach determined by injury type: neurapraxia/Grade I - conservative follow-up (complete recovery 6-12 weeks); axonotmesis/Grade II - conservative follow-up + physical therapy (regeneration 3-6 months); neurotmesis/Grade III-V - surgical repair. Surgical techniques: primary repair (end-to-end repair - sutured without tension, success increases if within 72 hours), nerve grafting (sural, medial antebrachial cutaneous nerve graft - for tension-free closure), nerve transfer (transfer of adjacent nerve fascicle to accelerate motor regeneration in proximal lesions) and nerve conduit (artificial nerve guide - for short defects <3 cm). Post-operative physical therapy, splinting and electrical stimulation applied. Prognosis depends on injury type, patient age, delay duration and nerve type; motor regeneration slower than sensory, prognosis poor in proximal lesions. Early diagnosis and surgery (within 3-6 months) improve functional outcomes.