Postoperative wound infection is the archetypal surgical infection because it follows a surgical procedure and requires surgical intervention for resolution. As with many infections, best results are obtained by prompt diagnosis and treatment, which is facilitated by understanding the risk factors. The most obvious factor influencing risk of infection is the density of bacterial contamination of the incision. This was recognized several decades ago in the wound classification system that divides all surgical wounds into the following 4 categories: clean, clean-contaminated, contaminated, and dirty. Clean wounds result from an elective procedure without break in technique that does not involve any area of the body other than skin normally colonized by resident bacteria. Clean-contaminated wounds result from a procedure such as elective bowel resection that intentionally opens the gastrointestinal (GI) tract or other colonized region such as the female genital tract but does not result in grossly visible spill of contents during the procedure. Contaminated procedures are those with gross spill from the GI tract or trauma and emergency procedures in which a wound has been created without normal antisepsis and sterile technique. A dirty wound is one that results from an operation in an area of active infection or previous bowel injury and leak. Among these categories, infection risk ranges historically, before modern understanding and practice of perioperative antibiotic prophylaxis, from 2% for clean wounds to 30% to 40% for dirty wounds when the skin is closed primarily.