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.Comment: 10 pages, 2 figures, talk given by C.S. at X Mexican Workshop on
Particles and Fields, Morelia, Mexico, Nov. 6 - 12, 2005 (to appear in the
conference proceedings
