The association between star-forming regions and X-ray emission was
discovered as soon as X-ray satellites became available, over 30 years
ago. However, understanding of the nature of this association really
started when X-ray images and the resulting source identification
could be obtained. We now know that essentially all young stellar
objects, T Tauri stars and protostars, are X-ray emitters, although
the case of the youngest, Class 0 protostars, is less clear. The
paper briefly reviews X-ray emission and absorption mechanisms, and
emphasizes the "solar paradigm" for the interpretation of the X-ray
emission, originating in the magnetic activity of young stellar
objects. The possibility of star-disk magnetic interactions in
protostars is also examined. In addition, feedback effects on the
circumstellar disks are presented: ionization, and the associated
coupling with magnetic fields; particle irradiation, and its
consequences for the origin of "extinct radioactivities" in
meteorites.