In recent years, the learning of algebra by all students has become a significant national priority (Moses & Cobb, 2001; National Council of Teachers of Mathematics, 2000). Algebra is considered to be a foundational topic in mathematics (Usiskin, 1988) and some have argued that an understanding of algebra is fundamental to success in today's technological society (Moses & Cobb 2001; Nathan & Koellner, 2007). Whereas the National Council of Teachers of Mathematics [NCTM] (2000) advocates that students should "develop their skill in...solving linear equations in the middle grades" (p. 39), many middle school and high school algebra students have been reported to demonstrate a lack of understanding when solving equations. Students have also been found to experience difficulty when attempting to represent and solve algebra word problems. The purpose of this study is to explore the nature of the algebraic understandings developed by students who learned to solve algebra word problems within the context of a structured diagrammatic environment. Three sixth grade students who had never before taken algebra took part in a twelve-week teaching experiment in which they learned to represent and solve algebra word problems. Students were given tasks which were similar in structure to those utilized by the Singaporean model method. However, the sequencing of the tasks used in this study differed from that of the Singaporean curriculum in that, immediately following their use of diagrams to model and solve algebra word problems, students in this study were introduced to the use of letters and numbers to accomplish the same. The tenets of Realistic Mathematics Education's theory of lesson design were employed in choosing the scope and sequencing of the tasks. Using APOS theory (Dubinsky, 1991), Simon and colleagues' (2004) activity-effect theory, and Gray and Tall's (2007) procept theory, the nature and development of students' algebraic understandings were examined. It was found that students developed four overarching understandings. Three of these understandings are prominent in the literature as being prerequisite to a students' algebraic development. Underlying and supporting students' development of these understandings were the following conceptions: an additive quantity can be treated as a process and an object (a procept), a quantitative whole is decomposable, and the parts of a quantitative whole are commutative. [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/en-US/products/dissertations/individuals.shtml.]