This qualitative case study examined the contribution a year-long professional development in the science of reading had on 10 first-grade teachers' knowledge of how reading develops and their instructional practices. Data for this research study was collected through a longitudinal interview process, from pre- and post-knowledge inventory assessments, and implementation notes from the organization providing the professional development. All interview data were coded and categorized to identify themes that emerged while the other two data sources were analyzed descriptively. The findings show the year-long professional development in the science of reading contributed to an increase in teacher knowledge on how reading develops. With respect to teacher knowledge on how reading develops, the emerging themes aligned with the theoretical framework of the Simple View of Reading. Additionally, the data revealed participants' understanding of the importance of explicit instruction while also showing their understanding of how small group instruction should focus on skill development. Regarding instruction, the findings highlighted participants' understanding of the need to focus on phonemic awareness, vocabulary, background knowledge, and syntax and text structure. As for assessment, the data revealed participants' knowledge with assessment grew with an increased understanding of screening, diagnostic, and progress monitoring assessment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]