The T Tauri stars

The observational aspects of T Tau stars are described, treating data in the optical, infrared, radio, X-ray and ultraviolet regions of the spectrum. Erratic flare-like behaviour is very characteristic of these objects which are pre-main-sequence stars, mostly on their convective tracks, and represent early phases in the evolution of roughly solar-mass stars. Herbig-Haro objects are discussed-shocked nebulae found in association with either visible T Tau stars or infrared sources deeply embedded in the parental dust clouds. It is argued that highly anisotropic mass loss, probably in unpredictable short-lived episodes, is found in the stars that excite Herbig-Haro objects. These exciting stars could well be the precursor to T Tau stars and display a fascinating array of astonishingly well-collimated phenomena.