Several activity related phenomena, such as X-ray emission, flares and
strong chromospheric emission, are present in T Tauri stars. They are equally
responsible for some of the observed peculiar characteristics of this
class of stars.
The launch of ROSAT opened a new era in the study of T Tauri stars recovering the previously known Classical T Tauri stars (CTTS) and locating a class of objects of which few were previously identified, the Weak T Tauri stars (WTTS). Prompted by the surprising IUE results, the study of X-ray emission points for the existence of extremely high levels of surface magnetic activity in these young stars.
We will review the impact of ROSAT observations in our understanding on the origin of X-rays and coronal activity. Further to that, we will also address a more detailed analysis in the X-ray spectral range of TW Hya a CTTS. This will be complemented by a summary of its properties in the UV, allowing for an overall picture of the activity in this star.