Abstract
The history surrounding the discovery of Herbig-Haro objects and their early interpretation is briefly discussed. Evidence which gave rise to both the reflection and emission hypotheses for their origin is reviewed, and observations which resolved the issue and pointed to the shock wave origin of the objects are summarized. Several theoretical scenarios are reviewed, including the shocked cloudlet, interstellar bullet, and focused stellar wind models. Selected observational highlights on Herbig-Haro objects in the infrared, radio, optical, and ultraviolet portions of the spectrum are discussed. The evidence implicates the Herbig-Haro nebulae with supersonic, bipolar mass outflows from T Tauri stars. Special emphasis is given to recent optical and ultraviolet work which suggests that the blue-ultraviolet continuum in the objects originates from the two-photon process, and new ultraviolet observations revealing H2 fluorescence emission in low-excitation nebulae are reported.
