Kai Behrend is a German mathematician. He is a professor at the University of British Columbia in Vancouver.His work is in algebraic geometry and he has made important contributions in the theory of algebraic stacks, Gromov–Witten invariants and Donaldson–Thomas theory. Perhaps, he is best known for Behrend's formula, the generalization of the Grothendieck–Lefschetz trace formula to algebraic stacks.