T Tauri stars are young pre–main–sequence stars and were first identified because of their variability. They emit excess radiation, relative to normal stars, at both long (infrared) and short (ultraviolet) wavelengths, and also emit at X–ray and radio wavelengths. Early models for both the excess radiation and variability in terms of strongly enhanced magnetic chromospheric/coronal activity were not very successful. Once it was realized that most T Tauri stars lie at the centre of gaseous discs (left over from the process of star formation), it became widely accepted that the UV excess is caused by continued accretion, and the IR excess by radiation from the disc. More detailed investigations of this model have brought the wheel full circle with the realization that T Tauri stars do have strong magnetic activity, and that many of their properties can be accounted for in terms of interactions between the stellar fields and the surrounding discs.