In a cross-society comparison, we assessed the state of mothers' knowledge of child-rearing and child development. The study included 1,077 mothers from five countries on four continents: Argentina, Belgium, Italy, South Korea, and the United States (U.S.) A criteria-referenced instrument, the Knowledge of Infant Development Inventory, was used to assess parenting knowledge after being adapted for cross-society comparison using item response theory and the alignment optimization approach for testing between-sample measurement invariance. Levels of mothers' parenting knowledge varied across the five societies and were associated with different sociodemographic factors and personal and nonpersonal supports. [This research was supported by the Intramural Research Program of the NIH/NICHD, USA, and an International Research Fellowship in collaboration with the Institute for Fiscal Studies, London, UK, funded by the European Research Council under the Horizon 2020 research and innovation program (grant agreement no 695300-HKADeC-ERC-2015-AdG).]