To investigate the formation and early evolution of stars, astronomers study the x-ray emission of T Tauri stars, which are young, solar-mass stars called pre-main sequence stars. Two Earth-orbiting x-ray satellites, the Rontgen X-ray Satellite (ROSAT) and the advanced Satellite for Cosmology and Astrophysics (ASCA), have discovered x-ray emission from young protostars, called Class I objects. Many T Tauri stars were detected as x-ray sources by ROSAT. X-ray luminosity functions and correlations with other stellar parameters can be studied and used to investigate the x-ray emission mechanism. From the ROSAT data hundreds of T Tauri stars have been discovered, some of which are located outside regions of ongoing star formation. Stellar x-rays also irradiate circumstellar disks, regions where planets may form, so x-ray emission data from T Tauri stars may also be used to investigate the formation of planets.