  We present a new approach to the dynamics of interacting particles with
reaction and diffusion. Starting from the underlying discrete stochastic jump
process we derive a general field theory describing the dynamics of the density
field, which we relate to an exact stochastic equation on the density field. We
show how our field theory maps onto the original Doi-Peliti formalism, allowing
us to clarify further the issue of the 'imaginary' Langevin noise that appears
in the context of reaction/diffusion processes. Our procedure applies to a wide
class of problems and is related to large deviation functional techniques
developed recently to describe fluctuations of non-equilibrium systems in the
hydrodynamic limit.
