Meta-analysis has become one of the most powerful and popular tools for investigating gender differences. Two meta-analyses of gender differences in mathematical tasks, Friedman (1989) and Hyde, Fennema, and Lamon (1990), are reviewed here. They provide evidence that these gender differences are declining over time. Since 1975, scores on the quantitative section of the Scholastic Aptitude Test echo this trend. A new meta-analysis of correlations of spatial and mathematical tasks is reported in this chapter. Sherman's (1967) hypothesis that a male advantage in spatial ability produces a male advantage in mathematical tasks has inspired an abundance of studies: these, along with factor-analytic and more recent psychometric contributions, form the core of this research synthesis. Both spatial and mathematical tasks have been categorized, and correlations within different categories are examined separately. For the most part, results are not consistent with the theory that spatially-based skill underlies mathematical thought.