  This paper given at the meeting on "Mapping, Measuring and Modelling the
Universe" presents three topics: 1) the study of the clusters and groups of
galaxies found serendipitously in the North Ecliptic Pole (NEP) region of the
ROSAT all-sky survey; 2) the highest redshift clusters found in the EMSS (up to
z=0.82) and the cosmological implications of their very existence; 3) the
gravitational lensing in the EMSS X-ray selected clusters of galaxies observed
by the Hubble Space Telescope.
