Decomposition of the diagonal is a basic method in the theory of algebraic cycles. The method relates the birational geometry of a variety to properties of the Chow groups. One recent application is that the Chow ring of a finite group can depend nontrivially on the base field, even for fields containing the algebraic closure of $Q$. Another application is that a very general complex hypersurface in $P^{n+1}$ of degree at least about 2n/3 is not stably rational.