  For macroscopic systems, the second law of thermodynamics establishes an
inequality between the amount of work performed on a system in contact with a
thermal reservoir, and the change in its free energy. For microscopic systems,
this result must be considered statistically, as fluctuations around average
behavior become substantial. In recent years it has become recognized that
these fluctuations satisfy a number of strong and unexpected relations, which
remain valid even when the system is driven far from equilibrium. We discuss
these relations, and consider what they reveal about the second law of
thermodynamics and the nature of irreversibility at the microscale.
