Teachers' ability to elicit and build on students' mathematical thinking during instruction is critical in order to support students' mathematical growth. An emerging hypothesis in the field is that the construct of a "learning trajectory" (LT) has the potential to support teachers in making sense of and using student thinking to improve teaching and learning. Early research on teachers' uses of LTs suggest LTs provide a framework for making instructional decisions and afford teachers with a means to focus on their students' mathematical thinking. However, little is known about the specific ways in which teachers use LTs throughout lesson planning, instruction, and assessment to focus on students' mathematical thinking. This study investigated five second grade teachers' uses of a LT using a multi-case study design. The ways in which the participants used a LT throughout three teaching cycles of lesson planning, implementation, and assessment were examined. From this examination, an initial framework was developed to describe different levels of teachers' uses of LTs. The study's findings suggested that the LT supported teachers in specifying learning goals and anticipating levels of sophistication among expected strategies. The teachers used the LT to pay attention to the processes their students engaged in as they solved tasks and it supported the recognition of important mathematical ideas that surfaced during instruction. The variation among the ways the LT was used by teachers highlight key aspects to attend to as we continue to study the ways in which teachers use LTs to focus on students' mathematical thinking. [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.]