  I briefly review the ''decohering histories'' or ''consistent histories''
formulation of quantum theory, due to Griffiths, Omnes, and Gell-Mann and
Hartle (and the subject of my graduate work with George Sudarshan). I also sift
through the many meanings that have been attached to decohering histories, with
an emphasis on the most basic one: Decoherence of appropriate histories is
needed to establish that quantum mechanics has the correct classical limit.
Then I will describe efforts to find physical mechanisms that do this. Since
most work has focused on density matrix versions of decoherence, I'll consider
the relation between the two formulations, which historically has not been
straightforward. Finally, I'll suggest a line of research that would use recent
results by Sudarshan to illuminate this aspect of the classical limit of
quantum theory.
