  We establish that boundary degrees of freedom associated with a generic
co-dimension one null surface in $D$ dimensional pure Einstein gravity
naturally admit a thermodynamical description. We expect the $\textit{null
surface thermodynamics}$ to universally follow as a result of the
diffeomorphism invariance of the theory, not relying on other special features
of the null surface or the gravity theory. Using standard surface charge
analysis and covariant phase space method, we formulate laws of null surface
thermodynamics which are local equations over an arbitrary null surface
paralleling local versions the zeroth and first laws and the Gibbs-Duhem
equation. This thermodynamical system is generally an open system and can be
closed only when there is no flux of gravitons through the null surface. Our
analysis extends the usual black hole thermodynamics to a universal feature of
any area element on a generic null surface. We discuss the relevance of our
study for the membrane paradigm and black hole microstates.
