Although preservice elementary school teachers (PSTs) lack the understanding of multidigit whole numbers necessary to teach in ways that empower students mathematically, little is known about their conceptions of multidigit whole numbers. The extensive research on children's understanding of multidigit whole numbers is used to explicate PSTs' conceptions of these numbers. A grounded theoretical approach leads to the development of a framework for PSTs' conceptions of multidigit whole numbers, and that framework facilitates description of their conceptions and their difficulties in the context of the standard algorithms. The framework also enables discussion of the PSTs' performance in other contexts. (Contains 15 figures, 4 tables and 10 footnotes.)