In his book A Brief History of Time, Stephen Hawking tells how he was told that every algebraic equation included in his text would halve his sales. He took a risk, included one, and produced a best seller. The equation, irrevocably linked to the name of Albert Einstein, is E = Mc2. Its rise to public prominence in the twentieth century came with the development of nuclear weapons and of nuclear power. We shall meet it soon, but there will be a few others, although the number to be understood and remembered is only six! We enumerate those in Section 9.13.