This study was designed to determine whether teacher portfolios can be validly and reliably assessed, to investigate the effect of an instructional tool on increasing the level of reflective thinking in elementary preservice teachers' portfolios, and to find whether electronic portfolios designed and assessed in optimal conditions represent sufficient quality to make them useful in practice. Presumably, teachers who can reflect deeply on their work and its impact on others can improve the quality of their teaching. This study sought to answer the following research questions: (1) Does the "Rubric for Evaluating Portfolio Reflective Thinking" ("REPORT") demonstrate sufficient validity and reliability for use in measuring reflective thinking in preservice teacher portfolios? (2) Do levels of reflective thinking in preservice teacher portfolios, as measured by the "REPORT", differ between students who have and have not received instruction using a Scaffolding Intervention Tool? (3) Do elementary preservice teachers' portfolio rationale statements and reflective essays, as measured by the "REPORT", show sufficient depth of reflective thinking to aid their growth as teachers? Data analysis indicated that the "REPORT" instrument used in this study revealed moderate levels of interrater reliability and demonstrated sufficient content validity to be used to measure reflective thinking in preservice teacher portfolios. Also, data indicated that members of the treatment group, who had received instruction in reflective writing, scored significantly higher on five of the six domains and on the total score than members of the control group, who had not received instruction. There was no significant difference between groups on the Planning domain. Analysis of the overall levels of reflection in the portfolios of both groups showed that a substantially higher percentage of preservice teachers in the treatment group (47%) wrote reflective statements that reached high levels of reflection than did the preservice teachers in the control group (6.7%). Mann-Whitney "U" comparisons supported the conclusion that preservice teachers with instructional intervention in reflective writing could demonstrate their own development in the areas of knowledge, instruction, and professional growth using more in-depth reflection than could preservice teachers who had not had this instruction. Implications for practice and further research are provided. [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.]