  Several aspects concerning the origin of the very strong X-ray activity of T
Tauri Stars (TTS) are still not well understood. Important new insight came
recently from the Chandra Orion Ultradeep Project (COUP), a unique 10-day long
Chandra observation of the Orion Nebula Cluster, and the XMM-Newton Extended
Survey of the Taurus Molecular Cloud (XEST). Based mainly on the results of
these two large projects, I will discuss our current knowledge about the
location of the X-ray emitting structures in TTS, the nature of their coronal
magnetic fields, inferences for pre-main-sequence magnetic dynamos, and the
relation between accretion processes and X-ray emission.
