Porcine circoviruses (PCV) are small nonenveloped DNA viruses containing a unique single-stranded circular genome. Previously, no recognized link was found between PCV infection of pigs and disease, and PCV was considered a nonpathogenic agent. Over the last 5 years, a "novel" PCV, designated PCV2, has been associated with various disease syndromes in pigs, primarily postweaning multisystemic wasting syndrome (PMWS). Pigs with PMWS have a variety of clinical signs, including debility, dyspnea, palpable lymphadenopathy, diarrhea, and pallor or icterus. Lesions associated with the presence of PCV2 in a variety of cell types include lymphohistiocytic to granulomatous interstitial pneumonia, hepatitis, nephritis, myocarditis, enteritis, and pancreatitis. The lesions of PMWS have been reproduced experimentally after inoculation of piglets with PCV2 cell culture isolates, although the full expression of the disease syndrome may require the presence of other agents such as porcine parvovirus or porcine reproductive and respiratory syndrome (PRRS) virus. Recent reports have linked PCV2 to other disorders in pigs, ranging from abortion and reproductive failure to "atypical" PRRS. Available data indicate high seroprevalence of antibodies to PCV2 worldwide. The diagnosis of PCV2-associated disease is based on the direct demonstration of PCV2 antigens or nucleic acid in affected tissues. PCV2 is now regarded as an important emerging pathogen. Although vertical transmission has been documented, the epidemiology of PCV2 infections is poorly understood, as is the role of the immune response in controlling or augmenting disease.