Skin and skin structure infections caused by bacteria involve the skin layers and underlying connective tissue. Cellulitis and abscesses are two of the most common manifestations of these types of infections. Streptococcus pyogenes (group A streptococcus) is the predominant cause of cellulitis, but Staphylococcus aureus should also be considered. S. aureus remains the most common cause of skin abscesses, and over the past few decades, methicillin-resistant S. aureus has become a leading pathogen. Less commonly, gram-negative bacteria and strictly anaerobic bacteria are involved, becoming important pathogens to consider in settings of bite wounds, aquatic injuries and exposures, surgical wounds, and traumatic injuries. Infections caused by Pasteurella multocida are commonly associated with dog and cat bite wound infections, and Eikenella corrodens is a hallmark pathogen involved in human bite wound infections. Vibrio species and Aeromonas species are aquatic pathogens that cause severe necrotizing skin and soft tissue infections in patients with underlying risk factors such as chronic illness and immunodeficiency. Mycobacterium marinum, another aquatic pathogen, typically causes indolent, chronic granulomatous skin infections. Strict anaerobic bacteria and other gram-negative pathogens such as Pseudomonas aeruginosa are important causes of skin and skin structure infections in patients with underlying immunosuppression and comorbid conditions. Recurrent skin and soft tissue infections often prompt further evaluation for underlying immunodeficiency.