In the first part of my talk I will discuss the variational principle for general relativity and some associated subtle facts not shared by other interactions present in the nature. In particular, I will try to arrive at the correct action principle for general relativity in presence of null boundaries. Possible generalization to Lovelock theories will also be presented.
In the second part of my talk, I will discuss the thermodynamics of null surfaces. In particular, I will show that three different projections of gravitational momentum related to an arbitrary null surface in the space time leads to three different equations, all of which have thermodynamic interpretations. Finally, I will also discuss about the recent works that I am involved in, as well as other works that I am pursuing in various other directions.