As the importance of mathematics continues to grow in our society, so does the question of how to effectively teach mathematics. Although the claim that teaching does make a difference seems obvious, the question of what makes mathematics teachers effective is not easily answered. Evidence from research shows that students' mathematics learning is influenced by the teaching they experience at school. Using case studies, five teachers were followed in order to explore their role in effective teaching. Specifically, the study explored the role of these five teachers in implementing the four components of effective teaching: (1) content, (2) discourse, (3) equity, and (4) connections. All the teachers were teaching the second half of a year long Algebra I course at a school participating in a mathematics initiative. Various types of data were collected to better understand the role of the teacher including: classroom observations, a teacher interview and questionnaire, and a student test given at the beginning and end of the course, along with a student survey. [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.]