The Cosmological Constant Problems (Talk given at Dark Matter 2000,   February, 2000)
  The old cosmological constant problem is to understand why the vacuum energy is so small; the new problem is to understand why it is comparable to the present mass density. Several approaches to these problems are reviewed. Quintessence does not help with either; anthropic considerations offer a possibility of solving both. In theories with a scalar field that takes random initial values, the anthropic principle may apply to the cosmological constant, but probably to nothing else. 
Steven Weinberg (Theory Group, University of Texas at Austin)