Grandmothers provide far more childcare than grandfathers do. The present study investigated whether, and to what extent, this gender gap was explained by grandparent couples' division of labor into market work and childcare. The analysis was based on panel data from two waves of the SHARE, comprising respondents from 10 European countries. Linear regression models yielded four main findings. First, the average trade-off between hours of market work and childcare within grandparent couples was one for six. Second, transitions to retirement narrowed the gender gap in grandparenting: if a single-earning grandfather retired, his share of a couple's total grandparenting hours increased by more than 50 percent. Third, controlling for couples' division of market work, the gender gap in grandparenting was smallest in the north and largest in the south of Europe, corresponding to a geographical gradient in the family-state division of caring responsibilities and the societal framing of gender roles. Fourth, in egalitarian and traditional countries, the effects of market work arrangements were relatively weak, suggesting the primacy of gender norms as a macro-level force governing the division of childcare in grandparent couples. (author abstract)