This longitudinal study examined the influence of prekindergarten teacher characteristics and classroom instructional processes during mathematical activities on the growth of mathematics learning scores in prekindergarten, kindergarten, and first grade. Participants attended state-funded and Head Start prekindergarten programs. Mathematical performance was measured in fall and spring in prekindergarten and spring in kindergarten and first grade using the Test of Early Mathematics Ability-3 (TEMA-3; Ginsburg & Baroody, 2003). Two dimensions of the Classroom Assessment Scoring System (CLASS; i.e., "instructional learning formats" and "concept development"; Pianta, La Paro, & Hamre, 2008) were scored based on observed classroom mathematics activities. Teachers provided information about their education and years of prekindergarten teaching experience. Research Findings: Instructional processes that included elements of the CLASS concept development dimension, such as discussions and brainstorming to encourage children's understanding, were related to growth of mathematics scores. Neither teacher characteristics nor instructional processes of the CLASS instructional learning formats dimension, such as using different modalities and materials, and learning objectives, were related to growth of mathematics scores. Practice or Policy: The findings extend our understandings of how instructional processes impact children's early mathematical performance. These findings may be helpful in increasing our understanding of the types of instructional processes that might be emphasized in teacher professional development specifically related mathematical activities. Professional development that focuses on the CLASS concept development dimension may be easier for teachers to remember and implement in their classrooms and, consequently, have a greater impact on mathematics learning.