  I suggest that classical General Relativity in four spacetime dimensions
incorporates a Principal of Maximal Tension and give arguments to show that the
value of the maximal tension is $c^4 \over 4 G$. The relation of this principle
to other, possibly deeper, maximal principles is discussed, in particular the
relation to the tension in string theory. In that case it leads to a purely
classical relation between $G$ and the classical string coupling constant
$\alpha ^\prime$ and the velocity of light $c$ which does not involve Planck's
constant.
