This study explored the relationship between mathematics teachers' participation in professional development and subsequent changes in their instructional practices. This professional development aimed to help teachers to implement high-level tasks through the use of the "five practices": anticipating, monitoring, selecting, and sequencing (Stein, Engle, Smith, & Hughes, 2008). Data were collected from teachers' participation with their school-based colleagues in modified lesson study cycles (MLSC). During these cycles, teachers took turns selecting, discussing, and reflecting on the implementation of high-level tasks (focus tasks). Specifically, prior to classroom instruction, teachers anticipated possible student solutions to the tasks and issues that might arise during instruction. Following classroom instruction, the teachers reflected on the lesson and how students actually engaged in the task. Audio recordings and meeting artifacts (e.g., teachers' anticipated solutions to the focus tasks) were collected. Data from the MLSCs were analyzed to determine teachers' level of participation and the key ideas that were shared in the MLSC meetings. Four teachers' classroom instruction was also investigated. These teachers were observed teaching high-level tasks, including the focus tasks from the MLSCs. Data from these observations consisted of observation write-ups (detailed accounts of the lessons) and lesson artifacts (e.g., lesson plans, representations of displayed student work). These data were analyzed with regard to the level of cognitive demand of the task before and during the lessons and the teachers' use of the five practices. Teachers' engagement in the professional development varied greatly. All of the teachers struggled to implement cognitively demanding tasks at a high level, and they used the five practices inconsistently and sporadically. Two possible explanations for the teachers' struggles are: (a) the teachers failed to consistently anticipate how students would engage in the task, and (b) the chaotic environment of the school negatively affected some teachers' participation in the professional development and their use of instructional practices. The results suggest that future professional development should focus on teachers' content-specific instruction, while also being conscious of and attending to the challenges they face in their particular teaching contexts. [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.]