Large scale structure of the universe
  How is the universe organized on large scales? How did this structure evolve from the unknown initial conditions to the present time? The answers to these questions will shed light on the cosmology we live in, the amount, composition and distribution of matter in the universe, the initial spectrum of density fluctuations, and the formation and evolution of galaxies, clusters of galaxies, and larger scale structures. I review observational studies of large-scale structure, describe the peculiar velocity field on large scales, and present the observational constraints placed on cosmological models and the mass density of the universe. While major progress has been made over the last decade, most observational problems in the field of large-scale structure remain open. I will highlight some of the unsolved problems that seem likely to be solved in the next decade. 
Neta A. Bahcall (Princeton University Observatory, Princeton NJ)