Abstract It is axiomatic that application of closure theory – the foundation of isotope-based thermochronology – requires an empirical diffusion model. It is therefore surprising that the majority of thermochronological studies have not met this requirement. The advent of the multi-diffusion domain (MDD) model transcended this limitation yielding both diffusion and age information via routine 40 Ar/ 39 Ar step-heating of K-feldspar. Observed correlations between age and Arrhenius spectra show that Ar diffusion occurs by the same mechanisms in nature as in the laboratory. Under certain conditions, these data permit the recovery of a unique, cooling history. The community reaction included some unproductive lines of argument but some stimulated refinements of the MDD model that benefited the development of thermochronology. The MDD model was recently applied to muscovite upon recognition that the same diffusion mechanism operates in vacuum step-heating as in nature. The advent of 40 K– 40 Ca closure profile dating opens up a new thermochronological approach. Initial results confirm that muscovite intragrain defects can restrict effective diffusion length scales in white micas from 10–100 s of microns. Our hope for the future of the MDD model is that it be subject to aggressive and sceptical testing by the community in which quantification is valued over assertion.