The classical Diophantine problem of determining which integers can be expressed as a sum of two rational cubes has a long history; it includes works of Sylvester, Selmer, Satgé, Leiman etc. and a recent work of Alpöge-Bhargava-Shnidman-Burungale-Skinner. In this talk, we will use Selmer groups of elliptic curves and integral binary cubic forms to study some cases of the cube sum problem. This talk is based on joint works with D. Majumdar, P. Shingavekar and B. Sury.