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 deg2 to r = 21. We emphasize the importance and power of spectroscopy for exploring and understanding the nature and evolution of structure in the universe. This paper preserves the substance and style of Margaret Geller's 2010 Russell Lecture presented at the May 2011 Boston AAS Meeting.
