 Despite its familiarity, however, the thermodynamic arrow of time raises many deep questions relevant to both philosophy and the foundations of physics. This entry concentrates on two of them. In contemporary parlance, they are each questions about grounding. (1) What grounds the thermodynamic asymmetry in time? In a world possibly governed at bottom by time-symmetric laws, how do the time-asymmetric laws of thermodynamics arise? (2) Does the thermodynamic time asymmetry ground any other temporal asymmetries? Does it account, for instance, for the fact that we know more about the past than the future? The discussion thus divides between thermodynamics being an explanandum or explanans. What grounds the thermodynamic asymmetry, and given the asymmetry, what does it ground?