This mixed methods study explored elementary teachers' (n = 73) experiences with and perspectives on the newly implemented CCSS-Mathematics at a high-needs, urban Professional Development School in a state where roll-out of the standards has been fraught with opposition. Analysis of the survey, questionnaire, and interview data reveals the findings cluster around: familiarity with and preparation to use the standards; implementation of the standards, including incorporation and teacher change; and tensions associated with enactment of the standards. Notably, the teachers believed in the merit of the standards but were constrained by their inadequate content knowledge, limited aligned curricular resources, lack of student readiness, and a perceived mismatch with ELLs. The results illuminate the professional needs of teachers during this critical time of transition and also add to the scant research in this area. [For the complete proceedings, see ED583989.]