Necrotizing soft tissue infections (NSTIs) constitute a spectrum of diseases characterized by a necrotic infectious process primarily involving the fascia and the subcutaneous tissue with relative sparing of skin and underlying muscle although both can be involved. The fulminant, widespread and rapidly progressive necrosis which occurs in NSTI is often associated with systemic toxicity and is usually fatal unless promptly recognized and aggressively treated. This chapter will review disease classification systems, diagnostic tools, and management strategies for NSTIs.