  We present a novel treatment of resonant massive particles appearing as
intermediate states in high energy collisions. The approach uses effective
field theory methods to treat consistently the instability of the intermediate
resonant state. As a result gauge invariance is respected in every step and
calculations can in principle be extended to all orders in perturbation theory,
the only practical limitation in going to higher orders being the standard
difficulties related to multi-loop integrals. We believe that the longstanding
problem related to the treatment of instability of particles is now solved.
