  Anderson localization is derived directly from the path integral
representation of quantum mechanics in the presence of a random potential
energy function. The probability distribution of the potential energy is taken
to be a Gaussian in function space with a given autocorrelation function.
Averaging the path integral itself we find that the localization length, in
one-dimension, is given by (E_{{\xi}}/{\sigma})(KE_{cl}/{\sigma}){\xi} where
E_{{\xi}} is the "correlation energy", KE_{cl} the average classical kinetic
energy, {\sigma} the root-mean-square variation of the potential energy and
{\xi} the autocorrelation length. Averaging the square of the path integral
shows explicitly that closed loops in the path when traversed forward and
backward in time lead to exponential decay, and hence localization. We also
show how, using Schwinger proper time, the path integral result can be directly
related to the Greens function commonly used to study localization.
