Recent observations demonstrate that outflows traced by Herbig-Haro (HH) objects can extend for parsecs from their driving sources, at least an order of magnitude larger than previous work. Some well-known HH objects such as HH 34 and HH 111 are verified to be parsec-scale HH flows. Up to now, dozens of giant HH flows have been reported in the Orion, Taurus, Cepheus and Perseus molecular cloud complexes. It thus appears that it is common for HH flows to attain parsec-scale dimensions. The ubiquity of parsec-scale HH flows profoundly alters our view of the impact of young stars on their environment. They provide a fossil record of the outflow behavior and mass loss rate from a young star over a significant fraction of the 10⁵ year outflow phase, and provide indirect clues to the accretion history of the forming star. Many parsec-scale HH outflows with multiple internal working surfaces and S-shaped point symmetry about the central sources show evidence for variability of ejection velocity and orientation of the source jets. Giant HH flows with longer or comparable lengthes to associated CO outflows, imply that HH flows power CO flows. Giant HH flows are capable of heating, ionizing, and dissociating cloud material over parsec scale distances and may play a role in the chemical rejuvenation of clouds and the generation of the turbulent motions observed in molecular clouds as well as the self-regulation of star formation.