The current debate about Yucca Mountain has raised a wide range of issues concerning geological repositories. While these issues are wider than those generally discussed scientifically, a basic consideration in all debates is the difficulty of predicting the performance of repositories over geological time. The question for consideration here, is how, and to what extent, geochemical modeling can contribute to this aspect of the debate? Geochemical modeling for Yucca Mountain is carried out by a number of organizations and research groups applying different approaches and levels of complexity. Regardless of the approach used, the viability of the modeling is dependent on the basic assumptions and data used. In this presentation, the importance to modeling of assumptions concerning; solubility controlling solids, sorption, kinetics, colliodal species, precipitation of mixed compounds, matrix diffusion, and coupling of these phenomena will be examined in detail. Even where a model can successfully handle many of these complex processes, an additional challenge, to demonstrating its applicability, is to independently verify and validate the approaches being used. Examples will be given of how modeling of other well understood natural systems could be used for this purpose. The scientific considerations outlined above suggest that very detailed geochemical modeling is required for repository systems, however, this can lead to the perception that the problem is too complicated to understand. When this latter perception is coupled with the many large tomes describing the Yucca Mountain site, without an integrated, concise, easily understood summary document, one can understand that stakeholders often feel that they are being blinded with scientific ‘gobbledegook’. Consequently, the challenge for regulators is to marry results from scientific studies with the safety concerns of a wide range of stakeholders. To meet these concerns, informed decisions concerning the viability of repository designs need to be based on a clear understanding of the total repository system. So, it is only where increased geochemical modeling continues to clarify the understanding of the overall system that further development and refinement is warranted.