Nonspecific uterine infections reduce the reproductive efficiency of cows and the profit potential of dairy farms. Fortunately, most cows do not develop severe uterine infections. The term uterine infection indicates that the uterus is contaminated with pathogenic organisms. Actinomyces pyogenes, either alone or with other bacteria, is often associated with uterine infections. When A. pyogenes was isolated from uterine fluids after d 21 postpartum, cows developed severe endometritis and were infertile at first service. However, the exact causes of uterine infections are unknown but are associated with several factors. Cows with dystocia, retained placenta, twins or still-births, and various metabolic disorders are more likely to develop metritis than are other cows. Aberrant immune function before and after calving seems to predispose cows to severe uterine infections. Few cows die from uterine infections, but cows with uterine infections are more likely to be culled for poor reproductive performance. Also, uterine infections can reduce milk production, and some treatments contaminate milk. Because they are nonspecific, uterine infections are difficult to prevent; attention to sanitation and periparturient hygiene, especially during assisted calving, may be the best defense. Evidence that aberrant immuno function predisposes cows to uterine infections indicates that methods for regulating immune function in periparturient cows have the potential for preventing or treating uterine infections.