It was discovered at RHIC that the quark-gluon plasma is very strongly interacting matter contrary to naive expectation. First I will explain how this conclusion was drawn with emphasis on the characteristics of the physics of high energy nuclear collisions. After the introduction, I will examine the interaction in the quark-gluon plasma in three ways: interaction measure, charmonia with finite momenta, and fluctuations of conserved quantities. In the presentation, I will mainly discuss physical implications rather than technical details.