The nature, evolution and effects of pre-main sequence stellar magnetic activity are explored through observations with the ROSAT and ASCA X-ray satellites, the Very Large Array radio telescope and computer modeling. X-ray spectroscopy reveals that the spectra of main sequence stars and T Tauri stars have peaks at 0.2 and 1.0 keV. The extreme classical T Tauri star XZ Tau also has harder emission out to 3-5 keV and shows long term variability in contrast to the flare activity of weak-lined T Tauri (WTT) stars. Protostellar X-ray emission is searched for from a large sample of very young stellar objects, with only one new detection, Ced 110 IRS 6 in the Chamaeleon I cloud. The X-ray luminosity function of protostars is roughly consistent with that of T Tauri stars but further observations and more accurate determinations of extinction are needed. The nature of dispersed lithium-rich X-ray stars is explored through radio observations, which indicate that the dispersed lithium-rich ROSAT sources near Taurus-Auriga seem to be genuine WTT stars based on their radio luminosities. WTT stars and ZAMS stars show a clear separation in radio and X-ray properties, with WTT stars having larger X-ray and radio luminosities, and also showing evidence for X-ray saturation. The ionizing effects of pre-main sequence stellar X-ray emission on the local cloud environment is examined through model calculations. Such emission may ionize small (<0.1pc) regions around each star, but is not a dominant source of ionization for the bulk of the cloud.