  The cosmic microwave background radiation (CMBR) is providing important clues
into the evolution of the early universe. In this Science Perspective, Joy and
Carlstrom discuss one feature of the CMBR, the Sunyaev-Zel'dovich effect, which
can be used to determine the abundance and evolution of clusters of galaxies,
thereby constraining key cosmological parameters such as the matter density of
the universe and its expansion rate.
