In addition to being fascinating astrophysical laboratories in their own right, Herbig-Haro flows have recently emerged as key phenomena in attempts to understand the complex and violent processes that govern the birth of low mass stars. This book provides an up-to-date overview of the major advances which have been made in the study of Herbig-Haro jets. On the observational side, Hubble Space Telescope images have revealed the intricate details of the shock structures in these supersonic flows, millimetre interferometry and infrared imaging can now probe the detailed structure of the molecular component of deeply embedded jets, and sub-millimetre and radio continuum maps now enable us to study the near environment of the optically-invisible driving sources of jets. On the theoretical side, detailed numerical and analytical magnetohydrodynamic models have now reached a level of sophistication that allows realistic comparison with observations and insights into the jet formation mechanisms. Similarly detailed observational and theoretical studies of the discs and winds of T Tauri stars are now gradually being linked to the Herbig-Haro flows, providing a first glimpse of how stars can be born by both accreting and ejecting infalling material.