The vision of K-12 standards-based mathematics reform embraces a greater emphasis on students' ability to communicate their understandings of mathematics by utilizing adaptive reasoning (i.e., reflection, explanation, and justification of thinking) through mathematics discourse. However, recent studies suggest that many students lack the socio-cognitive capacity needed to succeed in learner-centered, discussion-intensive mathematics classrooms. A multiple case study design was used to examine the nature of participation in mathematics discourse among two low- and two high-performing sixth grade female students while solving rational number tasks in a standards-based classroom. Data collected through classroom observations, student interviews, and student work samples were analyzed via a multiple-cycle coding process that yielded several important within-case and cross-case findings. Within-case analyses revealed that (a) students' access to participation was mediated by the degree of space they were afforded and how they attempted to utilize that space, as well as the meaning they were able to construct through providing and listening to explanations; and (b) participation was greatly influenced by peer interactional tendencies that either promoted or impeded productive contributions, as well as teacher interactions that helped to offset some of the problems related to unequal access to participation. Cross-case findings suggested that (a) students' willingness to contribute to task discussions was related to their goal orientations as well as the degree of social risk perceived with providing incorrect solutions before their peers; and (b) differences between the kinds of peer and teacher interactions that low- and high-performers engaged in were directly related to the types of challenges they faced during discussion of these tasks. An important implication of this study's findings is that the provision of space and meaning for students to participate equitably in rich mathematics discourse depends greatly on teacher interaction, especially in small-group instructional settings where unequal peer status often leads to unequal peer interactions. Research and practice should continue to focus on addressing ways in which students can learn how to help provide adequate space and meaning in small-group mathematics discussion contexts so that all students involved are allowed access to an optimally rich learning experience. [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.]