We show that passage over a potential barrier is associated with a pole of the scattering (S) matrix. Thus reaction thresholds associated with barriers are described in quantum mechanics by poles of the scattering matrix, i.e. chemical reaction thresholds are resonances. Furthermore, by varying the parameters of a potential energy function, such a pole may be transformed continuously into a conventional resonance associated with a particle trapped in a well between barriers. Thus there is no distinction in kind between the poles corresponding to the two physical phenomena. Trapped state resonances though tend to be narrower than barrier resonances.