  The study of magnetic activity of pre-main sequence (PMS) stars through their
X-ray emission is entering a mature phase. We review here two recent
developments. First, we present some early findings from the Chandra Orion
Ultradeep Project (COUP) relating to the age-activity-rotation relations of PMS
stars. COUP is a nearly continuous exposure of the Orion Nebula Cluster over 13
days and gives the most comprehensive view to date of X-ray emission from the
PMS population. We find that the activity-rotation connection that dominates
main sequence activity behavior is absent in PMS stars, although the
activity-age relation is present as well as links to stellar mass and volume.
  Second, we discuss investigations of effects of PMS X-rays on their cold
protoplanetary disks. It is possible that the the ionization, heating and
spallogenic effects of high energy photons and particles produced in PMS
stellar flares have significant effects on disk dynamics, heating and
chemistry. X-ray ionization may induce MHD turbulence in the disk which, in
turn, may regulate disk evolution and planet formation processes such as Jovian
planet migration.
