Use of the Classroom Assessment Scoring System (CLASS) is increasing across the United States as an important indicator of the quality of programs for young children. Professional development is required to facilitate teachers' understanding of the instructional behaviors upon which they will be judged. This study investigated the use of the CLASS reliability training process as initial professional development for preschool teachers participating in an Early Reading First project. Findings reveal that this use of the CLASS reliability training process did jumpstart changes in teachers' behavior across the domains of emotional support, classroom organization, and instructional support, though the domains in which individuals were able to demonstrate the most improvement varied by teacher. These findings have implications for preschool teachers' professional development as they strive to meet new standards built into many quality rating systems for programs in early childhood. (author abstract)