Infections occurring in liver transplant recipients result in significant morbidity and mortality. Factors influencing the frequency of posttransplant infections include pretransplant nutritional status, latent viral infections, and the degree of immunosuppression used to modulate the immune response to the allograft. Infectious agents may be introduced into the patient via the allograft, through infusion of blood products, and through intravenous lines, catheters, and drains. Infections also develop as a result of reactivation of latent viruses or by overgrowth or invasion by endogenous organisms. The intensity of the immunosuppressive regimen directly affects the frequency of infection. Infection may be categorized as bacterial, viral, fungal, or protozoal. The most frequent organisms include bacterial--enterobacteriaceae; viral--cytomegalovirus; fungal--Candida species and Aspergillus species; and protozoal--Pneumocystis carinii. Diagnosing infection requires the use of many different methods in combination, including routine bacterial culture, viral culture, and fungal culture. Histologic and cytologic examination may lead to rapid identification of some organisms. Specialized collection procedures such as bronchoalveolar lavage provide rapid access to material for culture and cytologic examination. Serum serology in conjunction with histotopic or cytologic evaluation is useful in diagnosing some infections, such as Epstein-Barr virus. New technology such as polymerase chain reaction allows detection of all types of infection at or before the onset of clinical symptoms. Rapid and early diagnosis of infection in this patient population can reduce infection-related morbidity and mortality.