  We show how to do semiclassical nonperturbative computations within the
worldline approach to quantum field theory using ``worldline instantons''.
These worldline instantons are classical solutions to the Euclidean worldline
loop equations of motion, and are closed spacetime loops parametrized by the
proper-time. Specifically, we compute the imaginary part of the one loop
effective action in scalar QED using ``worldline instantons'', for a wide class
of inhomogeneous electric field backgrounds. We treat both time dependent and
space dependent electric fields, and note that temporal inhomogeneities tend to
shrink the instanton loops, while spatial inhomogeneities tend to expand them.
This corresponds to temporal inhomogeneities tending to enhance local pair
production, with spatial inhomogeneities tending to suppress local pair
production. We also show how the worldline instanton technique extends to
spinor QED.
