Antibiotic-associated infection with the bacterial pathogen Clostridium difficile is a major cause of morbidity and increased health care costs. C difficile infection follows disruption of the indigenous gut microbiota by antibiotics. Antibiotics create an environment within the intestine that promotes C difficile spore germination, vegetative growth, and toxin production, leading to epithelial damage and colitis. Studies of patients with C difficile infection and animal models have shown that the indigenous microbiota can inhibit expansion and persistence of C difficile. Although the specific mechanisms of these processes are not known, they are likely to interfere with key aspects of the pathogen's physiology, including spore germination and competitive growth. Increasing our understanding of how the intestinal microbiota manage C difficile could lead to better means of controlling this important nosocomial pathogen.