With the adoption of new content standards, teachers are often left without adequate curriculum resources. This study examined how educators used their curricular resources to teach new mathematics standards in the USA. Analyses of open-ended survey responses from 257 teachers and teacher-leaders in Grades 3 through Grade 5 indicated that every educator reported supplementing their districts' or schools' primary curricular resources with other materials. These supplements primarily included resources found for free on websites and resources that claimed to be aligned to the new standards, but varied in terms of alignment to national standards for effective mathematics curriculum. Implications for this study include further research on how teachers make decisions regarding curriculum resources as well as increasing teachers' access to quality curriculum materials that can support students' mathematical learning.