  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 and spinor QED using worldline instantons, for a
wide class of inhomogeneous electric field backgrounds.
