  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.
