  From data in the present we can predict the future and retrodict the past.
These predictions and retrodictions are for histories -- most simply time
sequences of events. Quantum mechanics gives probabilities for individual
histories in a decoherent set of alternative histories. This paper discusses
several issues connected with the distinction between prediction and
retrodiction in quantum cosmology: the difference between classical and quantum
retrodiction, the permanence of the past, why we predict the future but
remember the past, the nature and utility of reconstructing the past(s), and
information theoretic measures of the utility of history. (Talk presented at
the Nobel Symposium: Modern Studies of Basic Quantum Concepts and Phenomena,
Gimo, Sweden, June 13-17, 1997)
