Grothendieck introduced the notion of a “motif” in a letter to Serre in 1964. Later he wrote that, among the objects he had been privileged to discover, they were the most charged with mystery and formed perhaps the most powerful instrument of discovery.1 In this talk, I shall explain what motives are, and why Grothendieck valued them so highly. These are my notes for a “popular” talk in the ‘W...