  Redshift surveys are a powerful tool of modern cosmology. We discuss two
aspects of their power to map the distribution of mass and light in the
universe: (1) measuring the mass distribution extending into the infall regions
of rich clusters and (2) applying deep redshift surveys to the selection of
clusters of galaxies and to the identification of very large structures (Great
Walls). We preview the HectoMAP project, a redshift survey with median redshift
z = 0.34 covering 50 square degrees to r= 21. We emphasize the importance and
power of spectroscopy for exploring and understanding the nature and evolution
of structure in the universe.
