In our everyday lives we have the sense that time flows inexorably from the past into the future; that time has a definite direction; and that the arrow of time points towards a future of greater entropy and disorder. But in the microscopic world of atoms and molecules the direction of time is indeterminate and ambiguous. Although entropy typically increases (in accordance with the second law of thermodynamics) in the microscopic realm the entropy can occasionally decrease. These fluctuations of disorder have profound implications for the operation of the molecular machinery from which living cells are built. This lecture is free and open to the public. Gavin Crooks, LBNL