  We discuss physical properties and the baryonic content of the Warm-hot
Intergalactic Medium (WHIM) at low redshifts. Cosmological simulations predict
that the WHIM contains a large fraction of the baryons at z=0 in the form of
highly-ionized gas at temperatures between 10^5 and 10^7 K. Using
high-resolution ultraviolet spectra obtained with the Space Telescope Imaging
Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) we
have studied the WHIM at low redshifts by searching for intervening OVI and
thermally broadened Lyman alpha (BL) absorption toward a number of quasars and
active galactic nuclei (AGNs). Our measurements imply cosmological mass
densities of Omega_b(OVI)~0.0027/h_75 and Omega_b(BL)~0.0058/h_75. Our results
suggest that the WHIM at low z contains more baryonic mass than stars and gas
in galaxies.
