  For more than a century, physics has known of a puzzling conflict between the
T-asymmetry of thermodynamic phenomena and the T-symmetry of the underlying
microphysics on which these phenomena depend. This paper provides a guide to
the current status of this puzzle, distinguishing the central issue from
various issues with which it may be confused. It is shown that there are two
competing conceptions of what is needed to resolve the puzzle of the
thermodynamic asymmetry, which differ with respect to the number of distinct
T-asymmetries they take to be manifest in the physical world. On the preferable
one-asymmetry conception, the remaining puzzle concerns the ordered
distribution of matter in the early universe. The puzzle of the thermodynamic
arrow thus becomes a puzzle for cosmology.
