The X-ray spectra of cool stars are rich in emission lines which are
important diagnostic tools of the physical conditions in their coronae.
Medium-resolution spectra of a number of active stars,
including RS CVn binaries, dMe flare stars and PMS objects, have been
obtained recently with the ASCA and BeppoSAX satellites. These data allow
addressing important questions such as the temperature and emission measure
distributions in the coronae of active stars, the elemental abundances,
and the time evolution of parameters during flares. We present
representative cases of X-ray spectra of active stars during both
quiescent and flaring conditions, and discuss the physical implications
of these observations. The examined cases include the RS CVn binaries
$\lambda$ And and UX Ari, the BY Dra eclipsing binary HD9770, and the
young stars AB Doradus and HD35850. We analyze the quiescent emission of
these stars (with special emphasis on the problem of abundance
determinations) and discuss time-resolved spectroscopy of flaring events.