The rate-determining step is dead. Long live the rate-determining state!

The concept of a rate-determining step (RD-Step) is central to the kinetics community, and it is basic knowledge even for the undergraduate chemical student. In spite of this, too many different definitions of the RD-Step appear in the literature, all of them with drawbacks. This dilemma has been thoroughly studied by several authors in the attempt to "patch" the drawbacks and bring the RD-Step to a correct physical meaning. Herein we review with simple models the most notable definitions and some challengers of the RD-Step concept, to conclude with the deduction that there are no rate-determining steps, only rate-determining states.