Tissue invasion by organisms following breakdown of local and systemic host defences is defined as infection. This chapter describes pathophysiology of infections with special emphasis on wound infection. Clinical presentation of common types of infections and their management is outlined. Various host defenses against infection, risk factors for increased infection, types of infection, Southampton wound grading system, source of nosocomial infection, complications of boils, differential diagnosis of abscess, high-risk anatomical areas of abscess, abscess versus cellulitis, complications of cellulitis in neck, etc. are described in boxes that can offer a quick glance. Boil or furuncle is the abscess in sweat gland or hair follicle that is caused by Staphylococcus aureus.