X-ray observations of star forming regions have turned out to be a
very powerful tool for the study of the low-mass star formation
process. The X-ray observations have revealed the existence of a new
class of low-mass PMS stars, the so called 'weak-line' or
'naked' T Tauri stars. While one of the main characteristics of the star
forming regions studied by the Einstein satellite was the strong
X-ray variability due to flares, comparatively few flares have been
detected by ROSAT. Since the weak-line T Tauri stars show on
the average a higher activity level than the classical T Tauri stars,
they are particularly well suited for subsequent photometric
studies of rotation periods. Here we review the results obtained so far
from X-ray observations of star forming regions, with particular
emphasis on the flare activity and on studies of rotation periods.