The goal of professional learning communities (PLC) is for teachers to come together to discuss and examine student learning and ultimately to make instructional changes that can lead to improved student learning. The formative use of assessments that are commonly agreed upon by this community of teachers is believed to enhance their improvement efforts. This study used a multi-case study qualitative approach to examine how elementary teachers working within a PLC apply the tenets and utilize the structure of DuFour, DuFour, Eaker and Many's (2006) PLC model to develop and use common formative assessments as a means for improving learning. Information gained from teacher surveys, interviews and observations of PLC meetings revealed that a collaborative culture, a shared vision that focuses on ensuring the learning of all students and a results orientation were the most prevalent tenets that influenced the teams' work and their ability to use assessments to improve students' learning. The teams varied in the extent they directly addressed the questions in DuFour et al.'s model but both teams paid considerable attention to a discussion of student learning and which students required extra support. The development and use of common formative assessments was a part of each team's PLC efforts but teachers also used other information about student learning to determine instructional changes. Results from this study support the theoretical proposition that the greater the internalization and application of PLC tenets and utilization of guiding questions, the higher the frequency and effectiveness of a team's ability to develop and use common assessment results formatively. This study's findings reinforce current literature and research regarding the importance PLCs and the formative use of assessment data can have on teachers' efforts to improve their instruction and ultimately their students' learning. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]