Tamagawa numbers and class numbers of algebraic tori were introduced and studied by T. Ono in the 60s, and were investigated by Katayama, Morishita and others. Recently progress has been made on computing Tamagawa numbers of Complex multiplication (CM) tori, due to its connections with CM abelian varieties, unitary Shimura varieties and counting abelian varieties over finite fields. In these two talks we shall discuss a few results of cohomological groups of some algebraic tori and applications of them to CM tori. This is based on joint work in progress with Pei-Xin Liang and Hsin-Yi Yang.