Many questions remain open as to whether, and to what extent comet properties were affected by thermal evolution in particular, since it is very difficult to reconcile expected results from thermal evolution modeling, and observations of comets’ parent bodies being processed, with observations that suggest comets remained unaltered for their lifetime. However, connecting comets to their origin requires that we are able to confidently distinguish primordial properties from artifacts of evolution. THEMISS is a modelling effort to face the challenge of constraining the thermally-induced physical and chemical variations of comet properties from the time they were stored in the Oort Cloud, the Kuiper Belt and the Main Belt, and during their active phase. It is build around three issues addressed below  that we believe will allow to make progress toward understanding the diversity among and within comets, providing constraints on their internal structure, key activity processes and their effects, and providing tools for disentangling primitive properties from evolved ones.