This overview summarizes the properties and mechanism of retroviral DNA integration and the consequences of the integration process to the infected cell and host. Integration of viral into cellular DNA is a regular and irreversible event, essential for virus replication, whose mechanism is becoming well understood. Many pathogenic consequences of retrovirus infection are due to side-effects of the integration process, including gene disruption, activation of proto-oncogenes, acquisition of oncogenes, and inappropriate expression of viral gene products. The properties and risks associated with these events are reviewed along with overall conclusions as to the importance of these to possible retroviral contaminants of biologicals.