From a worldwide perspective, porcine parvovirus (PPV) and porcine reproductive and respiratory syndrome virus (PRRSV) are the most common viral causes of porcine reproductive failure. A typical epidemic of PPV-induced reproductive failure is presented as an increased number of mummified fetuses and sometimes, entire litters are mummified. If infection with PPV is very early in gestation, the number of liveborn pigs may be further reduced as a result of embryonic death and resorption. During the acute stage of infection gilts and sows have few, if any, clinical signs, and it is unlikely that PPV is ever the direct cause of abortion. In contrast, a typical epidemic of PRRSV-induced reproductive failure is presented as a broader spectrum of clinical features including abortions, late-term dead fetuses, stillborn pigs, and weakborn pigs. In the later stages of an epidemic, there may also be an increase in the number of mummified fetuses, but their prevalence is likely to be far less than during an epidemic of PPV-induced reproductive failure. During the acute stage of infection with PRRSV, gilts and sows may have few, if any, clinical signs, or they may be severely affected and even die. This difference largely reflects the relative virulence of the strain of PRRSV causing the epidemic. A timely and reliable laboratory diagnosis of either disease can be made when appropriate tests are performed with appropriate samples. Vaccines are available for prevention of both diseases.