T Tauri Stars (TTS) are low mass (M≤ 1M⊙) pre-main sequence (PMS) stars. They are often classified into two main sub-classes: classical TTS (cTTS) and weak line TTS (wTTS). This classification is based on the strength of the Hα line (W(Hα ) ≥ 10 Å  for cTTS and W(Hα ) ≤ 10 Å  for wTTS) that was taken primitively as a good tracer of their magnetic activity (and youth).

The IUE has been contemporary to the rapid development of the research on star formation and this is reflected in the science carried out with it.

The first projects were devoted to study the magnetic activity of the TTS and the role that Alfvén waves may have in the acceleration of their very energetic winds (Giampapa et al 1981, Penston & Lago 1983, Calvet et al, 1985). In addition the emission line fluxes were converted into emission measures to model and analyze the structure of the atmosphere (Jordan et al 1982, Brown et al 1984).

The increasing evidence of the presence of disks around the TTSs (Snell et al 1980, Jancovicks et al 1983, Rydgren et al 1985, Bastien 1987, Beckwith et al 1990) as well as the realization that gravitational energy (accretion) is the most likely source for driving the outflows (Lada 1987, Cabrit et al 1990) led to analyze the UV excess of the TTS with respect to main sequence stars as generated in the accretion process (Bertout et al 1988, Simon et al 1990, Gómez de Castro and Fernández 1996). Moreover, the discovery by Ortolani and D’Odorico (1980) of UV emission from Herbig-Haro (HH) objects also made clear that the interaction of the collimated TTS jets with the surrounding medium is also susceptible to be studied with IUE.

Therefore there are several different physical processes associated with star formation that can be and have been studied by means of the IUE data presented in this Guide. The following is just a brief description of the UV spectrum of the TTS and a summary of the main findings on the TTS physics obtained from IUE data.