Research on fraction knowledge has revealed that prospective teachers' (PSTs) often lack fraction proficiency. The aim of this dissertation was to develop and test an intervention intended to increase PSTs' fraction proficiency by focusing on fraction comparison. I first adopted a five-dimensional framework to define PSTs' fraction proficiency. I argued that increasing PSTs' mastery of fraction comparison, conceptualized as the ability to use an extensive range of fraction comparison strategies and understand these strategies conceptually, can promote PSTs' overall fraction proficiency. Based on this argument, I developed a theoretical framework in which the mastery of fraction comparison is linked to the five dimensions of the fraction proficiency framework. Guided by this theoretical framework, I designed an intervention named Deliberate Fraction Comparison (DFC). To implement the DFC intervention, I developed a complete strategy space for fraction comparison and created a learning process for PSTs called Writing Deliberate Argumentation. Using a multiple-case study design, I implemented the DFC intervention with three purposefully selected PSTs in 17 two-hour group sessions over three months. I assessed the effectiveness of the DFC intervention on PSTs' growth in expertise in comparing fractions and development of fraction proficiency. I collected three datasets before and after the intervention, including PSTs' performance on using and justifying fraction comparison strategies, their achievement on a fraction proficiency test, and their responses to semi-structured interviews. By analyzing PSTs' pre-and post-intervention growth, I determined the effectiveness of the DFC intervention. The analysis results revealed that all PSTs increased their expertise in fraction comparison and increased their fraction proficiency. Specifically, all three PSTs expanded their strategy spaces for comparing fractions, deepened their conceptual understanding of a range of fraction comparison strategies, and became more confident in comparing fractions and teaching fraction comparison. Also, they achieved higher scores on a fraction proficiency test and developed their confidence in solving fraction tasks and teaching fractions. These findings indicate the DFC intervention can improve PSTs' mastery of fraction comparison and increase their overall fraction proficiency. The implications and limitations of the study and recommendations for researchers and practitioners are discussed. [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.]