2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1947I critically review a methodology of using clusters of galaxies as cosmological probes. The understanding of the abundances and spatial correlations of dark matter halos has been significantly advanced especially for a last few years. Nevertheless such dark matter halos are not necessarily identical to clusters of galaxies in the universe. This is a quite obvious fact, but it seems that the resulting systematic errors, on the cosmological parameter estimates for instance, are often neglected in the literature.10 pages, 8 figures. Invited talk at "Constructing the Universe with Clusters of Galaxies", IAP 2000 meeting, Paris (France) July 2000. To be published in the proceedings edited by Florence Durret & Daniel GerbalAstrophysicstext