  X-ray emission has played a major role in the recent identification of
several groups of young, nearby stars. The ROSAT All-Sky Survey (RASS) is due
to its spatial completeness an important tool for discovering stellar
associations and investigating their X-ray properties. However, it has
comparatively low sensitivity and poor temporal coverage, and is therefore of
limited use for variability studies. The situation is greatly improved by the
new generation of X-ray observatories, e.g. for coronal X-ray sources XMM has
about an order of magnitude higher sensitivity than the RASS and provides much
longer exposure times allowing continuous monitoring for more than 40 h.
  We demonstrate the capabilities of both the RASS by comparing X-ray
luminosity functions (XLF) of different young stellar associations, and XMM by
discussing one young very nearby star in detail, GJ 182.
