In order to understand past human behavior, it is necessary to identify and explain variability in the cultural materials resulting from this behavior. Chipped stone artifacts are the most ubiquitous cultural materials from the Middle Paleolithic. However, the interpretation of variability in these artifacts has been difficult. To address this problem, morphological variability in 1,146 Middle Paleolithic chipped stone tools, from four sites in the Iberian Peninsula, is examined in detail. This study differs from other analyses of Middle Paleolithic artifacts in emphasizing a quantitative investigation of both continuous and discrete morphological variability at the level of tool edges. These data permit analyses of the distribution of variability at the levels of individual edges, whole pieces, and assemblages. Patterns of lithic variability are also examined in the context of early Upper Pleistocene chronology and environment and compared with a larger population of Middle Paleolithic sites in Spain and the northwestern Mediterranean as a whole. For the assemblages studied, variability in edge morphology is predominantly continuous and normally distributed. Signficantly patterned relationships between edge attributes are restricted to cases in which one attribute limits, rather than determines, the range of variability in the other. These seem primarily based in the degree to which use, resharpening, and consequent edge reduction has taken place. Additionally, a dichotomy in patterns of edge use is suggested, associated with the extensiveness of use and modification. For whole pieces, most variability mirrors that of edges, suggesting that retouched artifacts are more the result of the extent and nature of the use of their edges than planned tools for which the maker had some form of "mental template." At the level of assemblages, temporal variability is minimal, while spatial and environmental associated variability is more apparent. These results are examined in light of the three most often proposed explanations for variability in Middle Paleolithic assemblages--style, function, and diachronic change. Subsequently, other aspects of Middle Paleolithic behavior--ranging from raw material usage to settlement patterns--are examined as potential sources for the patterns of lithic variability in the assemblages studied.