This study examines family and motivation effects on student mathematics achievement across 41 countries. The Rasch estimates of PISA mathematics test scores and questionnaire responses of 107,975 15-year-old students were analyzed via multilevel analyses. Students scored higher in richer or more egalitarian countries; when living with two parents, without grandparents, with fewer siblings (especially fewer older siblings); with higher family SES, more books, cultural possessions, or cultural communication; or when they had greater interest in mathematics, more effort and perseverance, and higher self-efficacy or self-concept. Family structure effects were stronger in individualistic or richer countries. Richer countries showed stronger family cultural communication effects, suggesting stronger, intangible resource effects. (Contains 4 tables and 1 figure.)