This is the written version of the author’s lectures delivered at the National Tsing Hua University, Hsinchu, Taiwan, in June 1992. Among other things, the ADM mass is considered and the positive mass theorem, proven by R. Schoen and S.-T. Yau and, alternatively, by E. Witten, is reviewed. Also, some open problems and conjectures concerning the notion of quasi-local mass are discussed.
For the entire collection see [Zbl 0870.00033].