The Universe Traced by Clusters

I 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.